Should I tell him how I feel?

Personally, it sounds like a sticky situation in a way. I think that if you care about both your best friends' feelings, you might want to wait until he would happen to break up with his girlfriend at this time in order to not lose her as your friend and also to not be part of the break-up. Sometimes people have a tendency to break up with one person in order to go out with someone else, which is not really a good situation because it's almost like a rebound situation, and once it happens, it is like a snowball effect where it just keeps happening - people breaking up to move onto the next person, that is. At any rate, that is my advice so far, but I wish you good luck with whatever you choose to do! :)

you should talk to him about how you feel, but in the same conversation, tell him why you think that it would be a good/not good idea to take the feelings to the next level.

You're experiencing sexual tension, and the only way to get rid of it is to talk about it to the person that you are tense with. Show him that you've been thinking about this for a long time by going into detail about why this perceived relationship would either work or not work.

trust me, you'll feel a lot better, and you won't loose him as a friend in the process. I know from experience.

If you cannot possibly do this, than the outlook is not good. you'll either push him away in frustration, push your girl friend away out of jealousy, or you'll push yourself on the guy and in the process make both of your friends hate you for ruining their relationship.

do it, that's all I can say, do it. better to feel awkward for a conversation, than to feel pain of broken relationships for years.

If it was me I would just go up to him and say "I like you and everyones telling me that you do is it true"?