We did it!
You are all right, and I knew it. I swallowed my pride and fear and just came out with it all last night. He tried denying it at first then I made it clear that I knew what was going on. His explanation is that they had a thing for years and now she contacts him from time to time, but that he has not had sexual contact with her for about a year.
I told him that after having a long relationship with her I found it hard to believe that he coincidentally stopped seeing her when I found out. He admitted he didn't expect me to believe it. He said that he didn't like the way he felt when he was with her because he felt like he was out of control - not like there was a second relationship in his life.
I told him I was asking for one thing - that he tell me if/when he is going to see her. He agreed and gave me permission to look through his communications to verify that he wasn't hiding anything.
I made it very clear that him telling me about seeing her for emotional or sexual reasons was not a deal breaker, but lying about it would be from now on.
Time will tell, but I think we are on the same page.
Thank you to everyone for the support and the tough love.