You've threatened, but not followed through, so obviously that doesn't work. And you certainly can't make him see your side of things. He's not considering your feelings because you're not giving him a chance to.
My advice? Drop the OW subject. Stop asking question, quit trying to trap him. The more you try to stop him from being in contact with her, the more likely he is not to see anything wrong with it.
I know you don't trust him. That's okay. But try putting your focus on other things...such as what is going right in your M.
