There's nothing you can really say to him that will convince him.
He's addicted to her.
They get over the OW by breaking off complete contact. They will grieve, hurt and be upset.
The key however, is that if he ends the affair and you go "back to the way things were" he'll end up having another affair. Generally affairs are symptoms that something's wrong.
There's nothing you can do to change him.
You, can, however, change yourself, which will change the dynamic of the marriage, and it might attract him back.
Get a life, enjoy yourself, pay attention to your own needs. Being a joyful, mysterious interesting person is attractive.
