Ive read on here many times, that being in an affair is very similar to being on drugs. The same "pleasure centers" in the brain are activated. The same responses to "help". And so on.
While someone is getting high regularly, what happens to the concerned people that try to help? They get shut out. They get abandoned. They become the ENEMY. Right now, thats YOU. There is nothing you can do to change him.
But. That doesnt mean that he has no reason to change. Eventually, the high runs out. Eventually, he will hit bottom. The real question is how much damage is he going to do on the way down? And will you still be there to pick him up?