system
A chat on subreddit /r/NoStupidQuestions
human
Chances are he feels unequal and like he’s not getting enough attention. He’s coming home from a school that gives a group that he isn’t a part of Special attention, then coming home and while I don’t doubt you make an effort, this post suggests you care more about other’s feelings than his. Give him a hug. Make sure he knows you love him.
gpt
Yeah, definitely feeling this. He doesn't seem anti-gay based on what OP said, just like he doesn't like parts of the inequality he sees, between how he's treated, and how LGBT people are being treated at school.

I think this is further shown by him asking why he can't wear a "straight-pride" shirt. He is feeling a lack of positive identity and inclusion in something that is getting people a lot of attention.

Maybe find some good things he can be proud of, rather than the reactionary "Well, I'm straight, so I guess if I wore a straight pride shirt, people should praise me too". Find some part of his heritage, some part of your beliefs/convictions/sacrifices, some part of his people, or one of his ancestors that did something good, and teach him about that, and why it makes him part of a special group he can feel pride in.