lol straight guys just say "nope, im straight sry" or something along these lines
he gets really rilled up and that question completely changed his mood lol
I know before I actually decided to come out to myself (aka being in denial), it would annoy me that people kept asking. Maybe he got the question often, and it annoyed him?
Regardless, I wouldn't expect him to give a mature response at 15, especially with the negative connotations being gay has in today's society.