I agree with BBT about not having these conversations in front of your son.
I also agree with having a talk with him.
In our case, we haven't really hidden anything from our kids. they see us/me on SI and I'm sure they've read the words "surviving infidelity" at the top of the page, but we've never told the kids about my A either. They do know that we had some big problems, and we explained to them that our marriage takes a lot more work that it might seem. Taking it a step further, we've talked with them about how important it is to communicate and also to face issues head on rather than letting them fester.
Being a good dad is being an honest dad; showing them how to take responsibility; showing them how to be a safe person; showing them that even when you make a mistake, you can own up to that, learn from it, and do what it takes to make sure it doesn't happen again.
Don't let your own feelings for guilt and regret get in the way of being there for your kids.