Well, I agree with slowmutant to a point - different kinds of different, but the boy probably wouldn't realize it and would be trying to find any explanation. It isn't unheard of.
The problem isn't so much if he is gay, but rather if he isn't but believes he must be, otherwise he wouldn't feel different. That sort of mindset can be damaging.
Perhaps, if he gets angry because he thinks you simply don't want him to be gay, have a gay person speak to him about it instead?
I shouldn't think it'd be a question, either, of him not knowing if he's gay or not - at thirteen, almost everybody knows, even if a small pocket might wish to deny.
If at 13 he's already gone out with girls, and then suddenly become gay - rather than tyically something gradual, it does seem a reaction to be seen in a more accepted difference. After all, aspies are subject to great stress in school environments, and accused of horrible things.
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Oh, well, fancy that! Isn't that neat, eh?