In my case, I have figured out that D was around from the beginning of H’s R with OW. It took me a long time to figure out what was going on and how much contact she was getting with OW too. I have decided not to confront for a number of reasons, but you will have to make the call for your own situation.
Here is what my IC and I worked out to tell my 3 year old D because of similar weird behavior and my suspicion that H had told her not to tell me about time spent around OW.
"It is ok to talk to mommy or not to talk to mommy about what you do when you are with daddy. Mommy loves you and daddy loves you. We both will always love you and take care of you. Some things may feel too big for you right now. It is not your job to worry about those things. Your job it to play and go to school."
If your S talks to you, try and take it in stride. Don't question, but let him know he can talk to you when and if he wants. Also, don’t pick up the phone and confront your H with whatever you hear from S. That puts your Sin a very tough spot. If you decide to confront about this, find a way to do it without evidence from your S.
Two other things I decided to do that may or may not be appropriate for you:
I have a few parenting/inspirational articles stuck on my fridge. I added the children of divorce bill of rights to it. Don’t know if H has seen it, but it is a good reminder for me, and that is really why it is there.
I also am trying to make it a point to mention any time D’s behavior seems really off. Sometimes I later find out about OW being around on those days. It may be optimistic to hope H makes the connection, but maybe it will sink in that her being around OW is not a good thing for her.
As far as H not talking to you, he is probably not feeling real proud of what he did. On some level he probably knows that was not a good move. My H is always cold to me and pulls back when he pulls this kind of sh**. It’s the guilt. Not enough to stop him, but it is there.
