To be honest, if he's bi, that won't make you terribly happy, because you won't have 100% of his attention, will you? And it sounds as that's what you'd like.
I think like others said, you need to sort out who you are yourself, first. Are you gay but using a girlfriend as cover? Or do you enjoy a female body too? It's difficult at your age I know, as even though now I know I'm gay myself, I know very well I was quite able to maintain an interest in a girlfriend at your age. I'm hardly surprised you're confused.
One thing you could try out, though you'd have to go about it with extreme caution, and that would be when you're both on your own some time, to get your friends views on homophobic people. If there's someone you know who is really badly homophobic, you could get your friends views on what he thinks of them rather than someone who seems quite tolerant. Having an offhand seemingly innocent discussion about homophobia, might give you a lot of clues as to his feelings about it and possibly warn you if he was liable to have a reaction you didn't want before 'coming out' to him!
Just another possibility you could think of trying some time, but pick the right moment when he seems in a very friendly mood!