Wow, I am so sorry to hear that you are going through this. I am hoping that you listen to my advice, because I wish that I would have gone onto a site like this and asked for advice when I was in this same situation. My ex husband was a complete liar and cheater. I found notes here and there, weird text messages, bizarre phone calls, yet I was in denial because he assured me he would never cheat. He said he was not a cheater and would never do that to me. Well I found out (by walking in on him having sex with our next door neighbor), and was devastated. Seven years later, I still can close my eyes and visualize every inch of what I saw that day. I later found out he had had several different woman throughout our 7 year marriage. He even had a child with one, who was married, although they never took a paternity test to prove it was his. What I am trying to say is, once a cheater, always a cheater. He is now re-married and guess what??? He cheated on her, too.
Only you know what you can live with, but I could not live with what I had seen with my own two eyes. I wish I would have realized what a scum he was earlier in our relationship, as we have a daughter together who now has a split family. However, she is my world and the only good that came of the relationship.
Trust me, cheaters always cheat again. He probably has done this throughout his life and just wont admit to it. Cut all contact and move on with your life. Your son deserves to have a wonderful role model in his life, or none at all- not one like this!