maybe the idea of boundaries will be useful to you.
He's going to do whatever he's going to do. He'll see the OW if he wants to. He'll come back exactly when he decides to.
All you can control is your own behaviour, and your responses to his.
A boundary in this situation might look something like this:
'I don't live in an open marriage. I don't have intimate contact or sleep alongside someone who is in a relationship with someone else.'
'I don't act like a wife to someone who has a girlfriend. That means no family dinners, no shared finances (or whatever it means for you).'
Boundaries are about you - protecting yourself, regulating your own behaviour. They're not about him. If he decides to continue his affair, and the consequences of that upset your son - he's doing that, not you. It might get ugly. I don't think you're going to get through this without your son knowing that something is wrong.
