Like Noah says, Will Byers is definitely 100% gay. He's meant to be gay from episode 1. This is quite a long analysis, as in several thousand of words long (not an exaggeration this took me two and a half hours), but I hope you like it and it's a good recap of Will's sexuality journey throughout the show for all those who already think Will is gay. And I'll talk about my thoughts for S5 a bit.
Just as a note before I begin analyzing the show: the Duffer brothers said that Will's sexuality would be clarified in Volume 2 and all that we saw in Volume 2 clearly was pointing to him being gay which I'll talk about later. They also said in a Collider interview that Will was speaking about himself through Eleven and that they used camera angles to show the duality of his speech. And they are the show's creators. Noah Schnapp said he's meant to be gay but he couldn't say so because of spoilers so if it was his interpretation why would he not be allowed to say it unless he's gay? But in this analysis, I'm going to try and prove to you why that's the case anyways without needing confirmation.
Right off the bat, we have the show's pitch/bible which describes Will as having sexual identity issues.
Now I've highlighted the things here relevant to his sexuality. I think the fact he has sexual identity issues is pretty self explanatory for how that applies to him being gay. It says he recently came to the realization he does not fit the 1980s definition of "normal" and considering this is the sentence after talking about his sexual identity issues, it's clear that's it about him realizing he's gay.
Other things of note is that it mentions he wasn't viewed as normal in the 1980s, a time where it was much less societally acceptable to be gay, and that normal is in quotation marks, showing he is normal but it just wasn't viewed as normal back then. It also says he is able to be himself uninhibited in fantasy, implying he can't be in real life, which I think could be a reference to him being closeted. Either way, this shows that he was planned to be gay from day one.
Now let's break it down season by season. In S1, we see that Will is bullied for seeming gay from the start of Episode 1 in which Joyce mentions his father called him slurs. He is repeatedly bullied for being gay throughout the series. Whilst being bullied for being gay doesn't make you gay, they're clearly planting the idea of Will's sexuality in our heads right from the start, especially relevant when you keep the pitch in mind.
Another thing of note is Will's conversation with Johnathan which was an allegory about being gay. Will's father didn't show up to take Will to the baseball and Johnathan points out he doesn't like baseball and that he shouldn't force himself to like things he doesn't like because people tell him he should, especially not his dad. Whilst this is about Will not liking sports, it can be read as Johnathan telling him that Will doesn't like things that "normal" boys like, but that's not a bad thing and that Will should just be himself. It's also important to remember that Will's dad is homophobic and that Will shouldn't force himself to like things his homophobic dad doesn't want him to like.
We also see a repetition of the word "normal" here. Johnathan says Will's dad is trying to force Will to like "normal things". We saw normal in reference to Will's sexuality earlier, so it's not too much of a stretch to say that this is about that too. Charlie Heaton also said at a conference that he knows Will is gay and that he wants to share that but can't and I get this is in reference to S4, but to me I view this conversation as Johnathan subtly telling Will that it's ok to be gay.
In terms of S2, we see less homophobic bullying. There are a few things to note though. When Will talks to Johnathan about feeling like he's a freak, Johnathan says that being "ordinary" is boring and that you'd rather be friends with someone like David Bowie (who was queer) than someone more dull (in this case Kenny Rogers). This is less clearly about sexuality than in S1 admittedly as it could be in reference to him feeling odd because of his Upside Down trauma, but I still think it's worth noting. Will also develops a close relationship with Mike and when he is mind flayed, Mike's voice is able to get through to him, showing he has a special bond with him.
Other things worth noting: the scene of Will at the Snow Ball. A girl comes up to Will and asks him to dance. Will starts to say no and looks at Mike instead of the girl. Mike indicates that Will should dance with the girl so he does. Ie he's liking something or rather someone because people tell him he should, not because he actually does. It said in a script that Will was looking at Mike whilst dancing but I don't know if this is a legitimate script based on this tweet. However it does mention that it's the season 4 scripts that aren't legitimate, not S2, so I'll post them here anyways.
Now onto S3. This is where Will's sexuality gets brought to the forefront a lot more. Will seems to be pretty jealous of Mike and Eleven's relationship and whilst it's partially about all of his friends having girlfriends, Mike clearly means something more. He was very excited to hear Dustin's girlfriend and he doesn't seem to have any issues with Lucas and Max being together. Will is being left out by everyone, not just Mike, which shows me that Will is interested in Mike. Will also looks quite pleased when Mike and Eleven break up.
This brings me to the iconic "it's not my fault you don't like girls" scene. I'm aware Mike's motivation in that scene may have been rooted in Will being immature, although that's not how Will took it. If you see the reaction on his face after Mike said that, it's much more personal than just that. It's also important to remember that Will has been bullied for his sexuality for years and now he feels as though his best friend has thrown him out of the closet before he ever got to fully accept he was in one.
Afterwards, Will looks at him and Mike and says "stupid" in reference to his own thoughts because Will knew deep down that he liked Mike but understands that it could never work out (not getting into if Byler is possible because this is about Will's POV). He rips up photos of him and Mike together. If this was a guy and a girl and one of them was insulted and started ripping up their photos whilst saying they were "stupid", it would be viewed as romantic, but I guess that's besides the point.
Will kind of falls into the background after this scene beyond the occasional neck tingle. I know there's other stuff you could mention, but I think with that I'm going to move on the season that solidified that Will is gay if the earlier seasons didn't do that as well.
At the start of S4, Will is acting weird and Eleven thinks this is because he has a crush. He's making a painting that he won't show Eleven so she thinks it must therefore be a romantic thing for a girl he likes. And yet Will brings the painting with him to the airport to give it to Mike. Also at the airport, Will looks overjoyed to see Mike and goes in for a hug with Mike which is not reciprocated leaving this sense of awkwardness. This could be platonic but remember that Eleven who isn't as socially observant thinks that this is because of a crush he has, ie a crush on Mike.
It is also mentioned that Will has a crush in his character description for this season, showing how they are very intentionally planting hints that Will Byers has a crush on Mike.
There are other things to note in this first episode. When a girl tries to hit on Will, Will is very clearly uncomfy with this action and he pulls his leg away and pulls a face, showing again that Will isn't interested in girls. This isn't about this specific girl too because it would be completely irrelevant to show that Will dislikes a random girl who is never brought up again, meaning it has to be there for a purpose. Will's hero this season is shown to be Alan Turing, a computer scientist who was also a gay man and persecuted for being so. He also has a poster for the song Boys Don't Cry and the lyrics seem to match up pretty well with him being unable to tell Mike his feelings and something I'll mention a bit later on.
I would tell you that I loved you
If I thought that you would stay
But I know that it's no use
That you've already gone away
This may seem random, but remember that every detail shown is most likely intentional. The poster isn't there to fill space, it's because the song has meaning to Will. I believe this is meant to refer to Will wanting to tell Mike his feelings but that he knows it's pointless and that Mike only cares about Eleven. If it wouldn't damage their relationship, Will would share his feelings to Mike which he clearly wants to do but is unable to because he doesn't want to lose his friendship with Mike but he's used to sharing things with Mike hence the van scene which I'll talk about in a minute. To reiterate, this post isn't about Mike, it's about Mike from Will's POV.
We see more of Will being jealous of Mike and Eleven in the scene at Rink-O-Mania. The theme of Rink-O-Mania in the sound track is called "In the closet" which whilst being a reference to Eleven crying in a closet could easily be about Will sadly watching as Mike skates away with his girlfriend. When Will and Mike fight, Mike says Eleven is his girlfriend and Will says what about us which compares him to Eleven in terms of his relationship with Mike which again could be platonic but the sheer amount of details challenge this.
Fast forwarding ahead because this is getting long, I'm going to talk about The Van Scene™. Remember that the Duffers said Will is talking through Eleven in this scene as you read the analysis, although I won't be referencing that as I'm trying to prove it with content in the show. Will decides that it's time to show Mike his painting. He says Eleven commissioned this painting but we know that's a lie because earlier Eleven said that Will would not show her the painting and that she thinks its romantic.
Why would Will lie about someone commissioning the painting, especially considering that person has romantic feelings? Because Will wants to tell Mike how he feels so he does it through Eleven. Will makes a speech about how Eleven feels about Mike but this is through the painting which again, Eleven did not commission. If it's not obvious enough, I'll change the pronoun from she to I and change Mike's feelings about her to Eleven just to make that clearer. Again, this is from Will's POV so don't come after me!
These past few months I've been so lost without you. It's just, I’m so different from other people, and when you're different, sometimes you feel like you're a mistake. But you make me feel like I'm not a mistake at all. Like I'm better for being different. And that gives me the courage to fight on. If I was mean to you or it seemed like I was pushing you away, it's because I'm scared of losing you, like you're scared of losing Eleven. And if I am going to lose you, I'd rather just get it over quick. Like ripping off a bandaid. So yeah, I need you, Mike. And I always will.
This is not about milkvan because it mentions being mean and pushing Mike away and Eleven was not pushing Mike away. She wanted to be closer to him which is quite the opposite. Anyways, after that speech Will smiles at Mike before turning away and crying into his hand because he didn't want Mike to see him crying. Why would Will be crying if this wasn't about Will knowing he'd just pushed Mike to Eleven whilst liking Mike himself. This brings me back to Boys Don't Cry, the song on Will's wall that I mentioned earlier.
I try to laugh about it
Cover it all up with lies
I try to laugh about it
Hiding the tears in my eyes
'Cause boys don't cry
I think that song could be referring to this moment. Will smiles at Mike whilst talking about Eleven because he's trying to hide his feelings and by crying into his hand, he is hiding the tears in his eyes. He tries to hide it because he doesn't want Mike to realize that he likes him because he's not "meant" to like Mike. The soundtrack in this scene is also of note as it is a blend of the songs "The First I Love You" and "The First Lie" because it is the first time Will has said his feelings to Mike out loud but he also lied about them by saying this was what Eleven felt like, meaning it was also the first lie he told Mike.
Finally for S4, we have the scene in Surfer Boy Pizza in which we have the final allegorical Will and Johnathan conversation to talk about. Johnathan noticed Will earlier and knows that Will was upset because he is in love with Mike and later on when Will is sadly staring at Mike being happy with Eleven, Johnathan takes the opportunity to tell him that if Will has anything he wants to tell him, Johnathan will be loving and supportive of him. The scene is meant to be Johnathan subtly telling Will it's ok to be gay, just like in S1. And I know I'm not meant to talk about actor confirmations, but this scene was confirmed to be about that by both Noah Schnapp and Charlie Heaton and they were directed to do it that way.
It's pretty clearly about Will's sexuality and lots of queer people with supportive people in their life can relate to someone telling them that if there's anything they want to share then it'll be expected. Will bursts into tears and hugs Johnathan. If this was about anything else, why would Will be so moved by it? Will feels deeply ashamed of his sexuality, referring to himself as a mistake for not being "normal", so he's very impacted by Johnathan letting him know that whatever he has to share, it's ok with him.
All of this to me shows that Wil Byers has been intended to be gay since Season 1. In terms of S5, I think that Will is going to come out based off of all this proof and since it's the last season and Will is clearly meant to be gay, it's going to happen now. I think we'll either get a quiet scene with a family member, especially Johnathan or a more explosive one with Mike where Will yells "because I love you" and then there's dead silence afterwards. Or both. Either way, I think Will's sexuality has been very well written and I have faith that the scene will be good, especially considering how much I loved Robin's coming out scene.
Hope this helps to convince your friends that Will is canonically gay. Don't know if this is what you wanted, but thanks for the ask anyways!