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I wouldn't tell your son anything about what you did that will cause them stress to think that his parents relationship is in jeopardy, I think you should put all of your energy into proving to your wife that what happened will never happen again you can't just say it she has to know in her mind and heart that you will not betray her in that way again. Tell your wife that you are human and you made a huge mistake that you regret making and wish you could take back. Also tell her that you are sorry for what you did and also tell her that you are willing to go to counseling if that is going to get your marriage back. What you need to do is have your actions match your words at all time especially when you are trying to regain her trust, also be honest even if you think she isn't going to like it this is VERY important part of getting trust back she has to know that you are willing to tell the truth and the whole truth.
Marriage counseling may be important also so that you can find out why you did it and what made you do it. Something was missing in your marriage or life that made you do this and for her to be able to even try to get over this you are going to have to find out what that was so the both of you can work on it together so that this doesn't happen again it's going to take some time to get things back on track and you will have ups and downs and it's very important not to involve you child into the situation at all, children tend to blame themselves when their parent can't get along or separate even when it isn't their fault. So any heated discussion you have with your wife should be done while the child is asleep or not around. The last thing you have to do is have no contact once so ever with the acquaintance you are trying to get your marriage back on track and you won't be able to do that as long as the acquaintance is around especially if the acquaintance knew you were married.