honestly, the moment someone starts talking nasty behind your back is the moment i think the relationship has entered a very dangerous level. everyone has their ups and downs, sure. but to trash talk someone they promised to talk to, love, and cherish shows a lot about who they are as a person. if you genuinely think this isnt him, i would say confront him directly about it and tell him EXACTLY how what hes doing is shitty. if there are members of his family you feel would be helpful, i would confide in them as well. but dont ever let yourself slip into talking badly about him. that will only start a cycle of anger and resentment.
in my honest opinion though, as someone from the outside with only a limited view on this situation, i would say leave him. especially if you dont have any assets with him.