Is it worth possibly ruining your friendship and closeness with him? How long have you felt this way about him? Is he happy with his gf? I had a friend that I knew for 2 and half years that I'm now dating, he asked me out several months after I broke up with my last bf, and had had a crush on me the entire time we knew each other. He only asked me out because there was mutual flirting, I had had time to get over my past relationship, and he was okay with having to deal with the loss of the friendship if it came to that.
Try and really evaluate the situation, you'll feel 100x worse if you say something and you end up ruining the way you guys relate. Remember that just because you're experiencing this inner turmoil right now it in no way reflects on your self worth. It's just something uncomfortable you're having to deal with, and can learn from. Date the girl if you're interested in her, if not try and focus on yourself and enjoying life. If you're not interested in her, think about the way you're feeling about this guy, she could be feeling the same about you, then you're just leading her on.
If he breaks up with his gf, announces he's also bi, and starts flirting with you and you're still interested, then go for it, but until then save yourself the heartache and keep your friend a friend.