Aside from intimacy, there's no absolute, positive way to tell for certain if someone is gay or bi (okay, an older person might be able to pick up certain indefinable signs that a younger person might not), but your mate has already hinted that he's fooled around with guys. The thing is, is he trustworthy? Was he telling the truth or only trying to find out if you're gay? Of course, if it's the latter, then he might be gay himself and as interested in you as you are in him (or at least hopes for some support). The trouble is that even if he's gay he may not be willing to admit it to anyone at this point, even you. And if you seriously think that he's likely to blab to everyone about what you've told him about yourself then you obviously don't think he's to be trusted just yet.
I suppose you could start a conversation with him on the subject without making any actual disclosure. You could say a closeted friend or acquaintance came out to you and see what your mate's reaction is. If he's positive and sympathetic, that's a start. He may confess all. [And for all you know, his mentioning of the gay incident that occurred while he was "tipsy" may have been a way for him to see how you felt about it, a starting off point for his own coming out. He might have been trying to see what reaction you would have to his own disclosure.] At least it will begin a dialog on the subject that may help you make a determination one way or the other. And his reaction to this imaginary person's coming out may also give you a hint as to how he'd react if he knew you were gay.
But be careful. When you're infatuated with someone, it's all too easy to imagine they're gay -- or at least that they're comfortable with their sexuality -- when they aren't. His saying that he got off with a guy, if true, is a red flag, of course. I can tell you that 100% hetero guys do not have sex with other guys even when under the influence. But whether he's comfortable with this or looking for a relationship is another story. He may not yet think of himself as being gay or even bi.
If he does come out to you and vice versa, don't immediately tell him about your deeper feelings for him. Give him time to digest the news. Even if he's gay, he may just want you for a mate. And he may not want to date you or anything else until you're both out of school. Yes, gay people can date each other while still in the closet, but of course it's easier when both are Out.
But the most important thing to remember is that Gay is Good. Have pride in yourself and all the rest will follow. Even if this mate of yours is not gay, not ready, or just not interested, there are many other young men out there waiting to meet you! Remember, when you're ready for intimacy, stay safe, and look forward to all the wonderful years ahead of you.
If you have more questions, let me know. Good luck!