How do I get my ex -boyfriend back.?

I have this friend we go out and have ocionaly but im still married.He has a girlfriend but I think he really dont care bout here.He also has other female friends that he be with.he says he would want to be with me but he dnt want to hurt me and I respect that ****.He want to change abd be monogomous and he needs help he said.How can help him out.And also how can I win him back.