Confessions: Will he really leave her and marry me?

I have been dating this guy for about a year and I recently moved in with him only to find out that he has a 7-month-old child with his ex-girlfriend and that he is supporting her financially. I called her up some time and she was straight forward with me that the child is his and that they are planning to get married. According to him, the child was conceived after a one night stand and that she trapped him into this. He says that he will not marry her even though the child is his. On several occasions, he has asked her in my presence to bring the child to him and we shall take care of him but she vehemently refuses and maintains that she is moving in with him – as he had promised. He has proven that he really loves me but I don’t know what to do if she moves in like she says she will. I really love him and don’t want to lose him. Please advise me.