If an open-R is OK, then when he starts seeing others, don't act like he is cheating. Accept it. Maybe see others yourself. Why not?
If an open-R is NOT-OK, then do not tolerate it. If he wants to be with you other than a friend and co-parent, he has to give up OW or any OW. Require one and only one more contact with OW in which you witness him come clean with her and tell her he wants no more contact with her. He can do this over the phone. If his behavior again becomes inconsistent, stop all non-business, non-kid contact. Do not accept more unless he does something to change himself -- therapy, medication, etc...
He CAN'T repeat the cycle unless you play your part in the cycle.
