You just need to set a boundary, and until he crosses it, he gets no convo from you.

1. When You have arranged for NO CONTACT with OW
2. When we are both seeing a GOOD FT we both LIKE together
3. THEN we can discuss US... until those things above happen... you get zero.

Re "dating" I would have corrected him immediately :

"When you 'date' while you are married, that's called infidelity.. no matter WHO the other person IS. It's cheating.. and I won't do it and I won't accept it from YOU either.

It's NOT dating.. its called cheating, its perverse and I don't want to do that and I don't want to be around anyone ELSE doing that... we are adults, we have a child here... this isn't high school anymore...

Say SOMETHING.. I can't believe he referred to "infidelity" as "dating"... ugh