Your son is an evil genius. But that's why you should keep certain info confidential even from family to avoid people conspiring against you. I won't vote YDI cause there is more to this FML, but you indeed set yourself up for that one. I'd also talk to your son much less cause he proved he's an informant for hire. I'd sanction him for that. Not as a means to necessarily punish him (like I said there's more to this FML), but to protect yourself and your pockets. Do indeed practice confidentially.

Don't know where you live, but in most US states with no-fault divorce laws, adultery doesn't generally come into play as a significant factor in the amount for divorce settlements anymore. (Your lawyer can explain your specific case in your country/state.) It's a legal process about equal or equitable division of goods. If one spouse spent significant money on an affair, that money is taken from their portion of the split or can be ordered paid back as part of alimony, just like any other wasteful personal spending can be counted against you (gambling debts, etc.), but the reason for the divorce is often irrelevant to the settlement. However, asking your kid to keep secrets from your soon-to-be-ex isn't really healthy behavior, and is going to make an already hard divorce harder. You put him in the place of having to choose between you and her, and he's already stuck in the middle, and he chose her. He may play both of you against each other either to punish you for the divorce, to get what he wants, or whatever else. He may also decide to side with one of you. Neither is particularly healthy. I guess you should try and do right by your kid as best you can in this and try to not make him and his loyalty/affection a pawn in your divorce games.

Never ever ever ask your children, no matter how old, to keep a secret from the other parent. It always comes out somehow & it puts them squarely in the middle of your issues. No sympathy here pal. Keep your girlfriend a secret from the kids if having a girlfriend is the problem....don't bring her around when your kids are there.

If you were already in the process of the divorce use that as an argument. Also, whether he hates you because you suck as a dad or because his mom turned him against you, at least you have a valid reason to give up your rights to him?

My mother made me keep her affair(s) a secret. I kept that secret and to this day it has an affect on me. It’s NOT ok to ask your child to keep a secret like that, no matter the situation between you and your wife. YDI. Period.

You're an AH. Don't bring your kid into your issues. He was right to tell his mother.