Sheesh everyone is getting pretty worked up....
OP - We meet people throughout our life, and we never know at what stage in a relationship these people are in when we meet them. This guy happened to be in a relationship when you met. Chances are he doesn't feel the same attraction/spark as you feel. If he did he probably wouldn't have gotten engaged to this other woman. He may also be a coward and is going along with the motions of his relationship. Either way, he seems like a douche for continuing to hang out with you. He is using you. You need to tell him ASAP that you have romantic feelings for him, that it bothers you he is letting things progress sin his relationship with his finance and still wants to hang out with you. Tell him straight up. Don't be afraid of what he will think or your friends for that matter. Be honest with him and don't be scared. Then, don't talk to him anymore. Do not contact him every again. If he breaks up with his GF and wants to date you for real then that;s would be ok, but otherwise you are wasting your time.