It is good that you know what you want. That is sometimes the hard part.
I don't think you can really reform someone who consistently cheats. I have known people who cheated and found polyamory and went the honesty route after that. However, I have also met people who will lie and cheat even when there is no reason for it. For example, one woman I dated was also seeing another guy. It did not bother me. One day she told me she broke up with him. However, it turns out that she didn't.
As for your current guy, I think he should realize that not everyone is into threesomes. It is only cheating if lying is involved.