I'll ask you a return question. What else do you think you should do? You've given your son an outlet. You've not brought him into an adult conversation, but let him talk and supported the mom (which most people struggle with - not just you) and you alerted your ex to your son's feelings.

Really - there is not much else you can do. Spinning it against her? Not really an option. The court is not going to react to someone who is divorced having sex while the child is with a sitter in another home.