Here is what I might say to him:

I can't begin to tell you how very disappointed I am to learn of this mess you've gotten yourself into. Your actions have profoundly changed the way I feel about what I have always admired in you. I want to make it absolutely clear that I don't approve of your behavior. I want you to understand that will not be participating in your lie. I will not cover for you, I will not bend the truth in any way to protect your secret. At the same time, I want you to know I cannot look your wife in the eye without telling her what you have done. Because of that, please don't expect me to go on with "business as usual". I can't. I won't. I think you are wrong to jeopardize what you have with your wife and family. I hope you will have the courage to put an end to your affair and to try to work through your problems with your wife. Meanwhile, just know that things cannot remain the same between us. I can't visit my sister-in-law and my neice and nephew anymore without tacitly becoming a part of your lie or else without telling your wife what is going on. Therefore, I strongly suggest you pull your head out of your zipper and demonstrate the courage necessary to come clean with your wife.