You're not going to get any sympathy from me. I've been cheated on, and it sucks. It took a year of therapy, nights of no sleep, losing weight due to no appetite, and medication for depression. And countless hours lost from being with my children because of all the above.
You can insist he tell his wife all you want, but he's not going to do it because he doesn't want to deal with the aftermath. You could tell her yourself, I suppose, but that comes across as pretty mean-spirited and spiteful.
Best to stay away from him. When, and if, he decides to pursue an open relationship/polyamory, maybe then you can approach him, but even then, you'd still be hiding the affair from his wife. And a relationship that begins with secrets is not likely to be successful anyway. Seek out and find those people who are actually available for a relationship, who can be open and honest, and strive to deal in honesty and openness in all your dealings with people and their emotions.