How do I get him back?

I had to continue my answer. LOL. Do you really want to be someone's second best? You deserve to be someone's first best and obviously this guy is not caring about your feelings. Back off a little if its not easy for you to be his friend. My best advice would be to let this guy dig his own hole and find out that you're not going to be his "plan B." I wish you all the best.

The one thing that you should not do is act like you don't care (in front of him) then he will think that it really is ok with you if you just be friends. Flirt with other guys around him but not to the point where it seems too slutty, just enough where he feels he needs your attention. That fact that he does not love her gives you an advantage. Their relationship may not even last that long anyway, it may seem like it will though. Get a positive kind of attractive feelings from him and then move on to a kind of sexy appeal.

Sounds to me like he's playing both of you. Why is he with her if he's not in love with her? Your best bet would be to shrug him off and move on. This guy is no good at all.