Help! I think I like my guy best mate!

I have been friends with him for a while and I don't know what to do. He has a girlfriend, I've never met her but I would never take a man from another girl. I just can't stop thinking about him and he's kind of stopped talking to me as much since moving away. I think I just miss him but I've never had this feeling with other friends before. I need help to get him out of my mind!!