I agree with Sponge Bob. You have to sit down and have a long
useful talk. If you cant there is your first problem. This is where trust comes into play. You have to becauseful about what you do. If you show him that you dont trust him he will probably be more inclined to cheat, itll be his way off rebeling. He'll probably figures, she thinks Im going to cheat anyway. I aught to just do it.
Weak excuse, but chances are thats what will happen.
Try to figure out a way to talk to him with out making him feel like a target. Dont bring up cheating directly, but kinda try to "hint around the bush" so to speak. The only person that can change himself is him. You cant so dont even try. If you do youll just make things worse. Someone who cheats, shows nothing but disrespect for the other person.
Good Luck with your situation.