I go back and forth on this a lot, because the Dingles as a whole would be Hufflepuffs. (The mascot is a badger, the house values loyalty, and the whole affinity for plants and animals thing. Not to mention the Dingle farm aesthetic fits what I presume the Hogwarts common room to be like — close to the kitchens and warm and cozy.) Of course, there would be outliers — like Cain, possibly.
But Aaron himself is very definitely a Gryffindor, especially when he was younger and even more hard headed and impulsive than he is now. He’d be scared of his feelings (both about his sexuality and, perhaps, Gordon), but not afraid to do the right thing, even though he might end up doing the wrong one in the name of love and friendship. So of course the hat recognises this and before he can even place it on his head, it yells, “Gryffindor!”
Later, a second year Robert will find a first year Aaron sobbing in an alcove that has become he favourite reading/fuming nook and he’ll ask him what’s wrong. It’ll take some time, but he’ll eventually work it out: Aaron doesn’t think he’s brave enough to be a Gryffindor. This is where Robert will tell him that the houses aren’t just about who you are, but also about who you want to be. “So if you’re not feeling your bravest right now. That’s alright. You can try and be brave tomorrow.”