I think you should be honest with your son in telling him that unfortunately his dad was there and now knows that they know. Keeping secrets like that gets no one anywhere and is going to put them both in an awkward situation and only make their relationship worse. As much as I would hope my boys would understand what I went through and disapprove of their father's behavior if they ever found out (they're very little) I wouldn't want it to ruin their relationship because as much as my H is an ass, aside from his affair he's also a good dad. I just think that's a heavy burden for your kids to carry and not be able to express their feelings to him and work through them. It'll suck for your H to hear how his behavior has now affected your kids as well but he'll just have to deal with the consequences of his actions.
I definitely agree with you that they don't need the details though.
However, you know your situation and your kids better than anyone, and a mother's intuition is usually right!

I believe the only healthy way to move forward as a family is to be open and honest about what has happened. Not talking about it and continuing to keep it "a secret" just makes it that much more dysfunctional. I assume since you were reading it together you and your cs are still together so why not celebrate the fact that for better or for worse actually happened and you are still a couple and working your way through it?
Maybe it was easier for me because I didn't have the choice to tell my kids or not because the OW threatened to tell them herself in her way, publicly on fb, so we had no choice but to tell them so they heard it from us first, but that begs the question, how did your children find out?
My heart breaks for you that they've been processing this on their own without being able to turn to you and I'm glad your son was brave enough to finally put it out there

I agree that you do need to talk with your children. I think you should talk with the three of them first, just you. I'd keep it simple. They
already know, it's not new news.

But your H needs to talk to them as well. I think I would do this though in a session with a T or a MC though. Your H will have to face the consequences of his actions. But it is still an issue that is primarily between you and your H.

Good luck.

Thanks all of you for your thoughts on this. I hate keeping secrets, it is not in my nature. This is why I told my H when he asked why I was crying. He did not read the conversation as he was working on his own computer at the time. I think he felt awful when I told him they all knew as he has had it easy up until now when we both thought noone else knew.

I've not had a chance to really talk to him about it yet as he is very busy with work and this happened after midnight last night. We should though and I hope he will talk to me as he is not one for sharing his feelings....

@Hope: they found out by guesswork from our behaviour as far as I can tell and at the time, so 20 months ago. As I said they know no details, what happened, how long for, who with etc.