I hope you get some more helpful and informed responses, but didn't want your post to just vanish!
Firstly, I think children do start to have an awareness of their sensuality at this age, but it isn't necessary set or fixed. I'm straight, and always knew that without realky questionning, so why would it be different for someone gay? There's no reason to think the puberty would change this.
That said I've got gay and bi friends that experimented with different relationships during their teens before settling down. So, it is possible he might yet decide that he's not gay.
I imagine it might have been really hard for him to tell you about this. You must have an excellent relationship. I think the most important thing is that he knows he has your unconditional love and care - that this doesn't bother you. That's a strong foundation to survive any bully from.
Hopefully it all blows over in school, these things do. It's really quite uncool to be homophobic these days, so he might well find others have his back. But if it doesn't die down, the school should support.