What do I do when a guy with a girlfriend likes me?

My friend and I are very close, almost like brother and sister, so it really shocked me when his buddies told me that he was practically in love with me and couldn't stop thinking about me. I confronted him about it in my joking fashion and he admitted that he found me very attractive, but this bothers me because he has a girlfriend that goes to a different college. Since then, we haven't really seen eye-to-eye and I feel that our friendship is suffering. My feelings for him changed as well, when I realized that I really really liked him. I am not the type of tramp that goes off stealing other girl's boyfriends, so I would never pursue him in any nature of that sort while he's with his girlfriend (who I met and is very nice). So far, he has ceased talking to me but always goes through my friends to see how I am doing. His friends tell me that he doesn't want to be near me in case those feelings he already has become stronger and he doesn't want to slip up (I will say it now, he's not the typical guy that plays girls up. He's in a really hard place right now.) I will bite the bullet if I have to against those feelings, just anything as long as I can get our closeness and friendship back, but I can't say the same for him. What should I do?What do I do when a guy with a girlfriend likes me?
Wait a few months, just pretend that nothing happened or that you know. Don't lead him on, and just be a friend. He will get the idea to just be friends. He may break up with his girlfriend, even then...if you want to date him wait a few weeks after the break up. What do I do when a guy with a girlfriend likes me?
Think about it.... how would you feel??? Maybe you should tell her. If she's with someone that can hit on her best friend is he worth being with her???