LGBT: very awkward thing going on?

If he's really your best friend, just try being straightforward about it. Tell him you've noticed some behavior from him (no need to be specific) that makes you wonder if he thinks he might be gay, and if he wants to talk about it. See where the conversation goes from there. If he mentions being attracted to you, you can just tell him that you care about him and you're flattered, but you like girls. Good luck.