I would say to tell him how you feel and tell him that him calling you and then staying with this female hurts you, tell him you want to be with him and you care alot about him but you will come second in his life. Ask him if he thinks the two of you have a chance. It sounds like he has made several bad choices and that if you can't stay faithful to you then he should let you go so that you can find someone that will treat you like gold. As far as fighting for him I don't know how much fighting you could do he has to want to be with you shouldn't have to prove that your the right person for him as he did says that he think the two of you are better together then he needs to tell this girl and come back to you but you can't make him do that, if you want an honest relationship with him then you ahve to communicate your feelings to him and let him make up his own mind as to what he wants.
If you feel he owes it to you to get you another chance then tell him that and don't beat around bush come right out and say that. If he can not stop seeing the other female then I would let him go and not wait for him until he is free from any kind of relationship and can give you all of him. If you have anything to add or still need my help click reply.