Have you told him two wrongs does not a make it right? It sounds like revenge to me. I would tell him that if after he cheats it makes him feel better than maybe that is a big red sign that it's time to move on. BUT if he does it and only feels remorse and truly repents then he might understand where you r coming from, but he reallly needs not even test the water if truly he wants to make the M work.
It sounds like he is way to angry about the situation and wants instant relief.