If I were in your shoes, I would continue to detach and just leave him be. The more you try to pin him down about the ow, the more likely he will be to continue working w/the ow. The less you say about her, the better. If he truly wants to work on the relationship, he will have a lot of heavy lifting to prove it to you. He needs to earn your trust and respect and that is on him to do so.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
