If YOU want to give it one last hurrah....a sort of AFTER THE LAST RESORT technique......
Tell him what you want, exactly...figure it out and say exactly what you mean.You do not tell him more than twice.
Reward good behavior, do not reward his bad behavior. I think sooooooo many of us on the board inadvertently reward bad behavior. I do it. Don't hang on the phone with him. He's manipulating you.
Expect prompt compliance. Don't give him extra time to work it out. If he wants back, he leaves her. Now. Or whatever YOU decide. Your terms, as your friends here have said.
Ignore undesirable behavior. Don't 'reward him' by getting into it. Hang up the phone (or whatever is appropriate).
If this works for you, there are a few more things you can do.
I'm not saying you 'should'. This would be long hard work. Only you know what you have inside you.
sg Love is PATIENT, love is KIND, LOVE never fails / DB since 2001