I think you should tell him and share all the information that you have. If you can get his work email that would be the best thing but you could also send him a letter to work. He has the right to know what the truth about his life is and his wife has the right to have her marriage be dragged through the wringer.
You don't say what your proof is but if it's phone records or texts or something, copy them and send them in the email. Give him everything you have, offer to meet with him and then see what happens next.
My husband initially lied about who the OW was married to to try and protect her marriage from me--it was pretty much the first thing I did was to phone who I thought was her husband. He had sent her a NC email on DDay and ironically, her husband found it 10 days later in her trash folder. So, I figure if that isn't karma I don't know what is. Good luck!