If he has another girlfriend right now regardless what you think of their relationship there is nothing you can do or should do. If you think he may not like you at this point - if you try to break his relationship up he currently has he will despise you even more. It is best at this point that you move on, and quit wasting your time on someone that is in another relationship and probably won't want to be with you anyway. There are so many others that will want and respect you if you let them. Move on.