“...And you?”
The answer should be pretty clear to Erik—Peter is there with Mystique simply because he’s the only member (other than Kurt, who’s busy elsewhere and who Erik doesn’t know about) who can actually travel to where Erik is. From Erik’s perspective, Peter isn’t there to add onto Mystique’s little monologue in some grand way—he’s just there to get her to and from places during this battle.
So why ask Peter this? What purpose does this question serve Erik other than wondering about that somewhat strange, energetic kid who broke him out of jail ten years ago? Why ask him why he’s there after a heartfelt speech about family, if not for confirmation of a decade-long suspicion—“My mom once knew a guy who could do that”—that maybe there’s some connection between them? Erik had to have asked about Peter at some point between the Pentagon and Paris. “Where did you find him? Is he a student? And how many people am I friends with that I apparently knew this kid’s mom?? Wait, his uncommon last name is the same as my ex’s?...How old is he again?”. It wouldn’t be hard to connect the dots after a little questioning. Between this and the look in his eyes after Peter begins saying “I’m your—“, there’s no way Erik didn’t know at that point.
Erik knows who his son is and he isn’t disappointed in who he is like Peter fears—he loves him because that’s his child, even if that child is all grown up and doesn’t really need him anymore. The only reason Erik hasn’t said anything is because he thinks Peter wouldn’t want anything to do with him, and he can’t risk losing anyone else even if he is accepted. Neither of them are willing to open themselves up for potentially experiencing the pain of rejection from the other—it would just be a son being abandoned for the second time and a father effectively losing yet another child—but it means they’re also closing themselves off from a much-needed familial connection.
All that to say—dang it, Fox Studios. This was such a good arc and it was wasted, and I will forever be sad about that.