You need to tell him loud and clear that you do not have feelings for him and that you never will. Don't even make this about the fact that you have a boyfriend. Make it about the fact that you do NOT have feelings for him. It sounds to me that even if you were single you wouldn't go for this guy. But by you telling him that you are happy in your relationship and discussing the relationship with him, he's probably thinking that the only thing standing in his way is your boyfriend. He probably is hanging on, waiting for the relationship to end, and who knows, could possibly do something stupid to try to break you up.

You letting him touch your stomach and kiss you most likely gave him the wrong idea (even though you just did it to not hurt his feelings) Don't give mixed signals, give very clear signals that it's not going to happen. Try to hang out with your friends and family and keep yourself busy so you don't feel lonley.