Back off. Find interest in another guy, than him. It doesn't matter if he's your best-friend, you should easily find interest in someone else. If you tell him how you feel, then that will just put you, him, and his girlfriend in a very awkward position, but if he somehow has feelings for you too, then that would put him; not you, in a difficult situation, because he wouldn't be able to decide who to be with. So, just back off. Always be there to support him though.
Since he has a girlfriend it is best to wait because that is like someone trying to get with your boyfriend you wouldn't want that :/ I agree with blackangel it doen't matter if he is your bestfriend it will cause some problems