OP there are a lot of children who don’t know that their father isn’t their father. It’s far more common than you think but generally it’s because of donation/infidelity. In your case you had a waste of space for a fling who pissed off as soon as you fell pregnant, and your now ex stepped up and brought your child up as his own.
You’re not the first and you won’t be the last and unlike others, I’m not sure that it matters that he didn’t know. I know too many people who knew their biological father wasn’t their biological father and who felt different growing up, as if they weren’t a part of the family, even though they were brought up exactly the same as the other children.
It’s too black and white to say that the child needs to know growing up, there aren’t actually any right or wrong answers, because what works for one child doesn’t work for another and so on.
These days people place huge emphasis on medical history etc, but reality is that if this man buggered off without a trace, the medical history is never going to be known, and it’s entirely possible it will never be needed.
Your existing husband is a cunt, and I would tell your son and then I would tell him to get the fuck out and never darken your door again. I hope you don’t have kids with him, because if not I would get the divorce sorted and never see or speak to him again.
When you explain to your DS I would do it in such a way as to say that when you and his father (it takes more than biology to make a father) got together, it just happened that you clicked as a family and the time never came up, even less so when you had children with him.
Ime the people who take these things the hardest are the people who didn’t have good relationships with the non biological father in the first place. It’s those people who feel robbed.
I’m in my 50’s, and if my my told me now that my dad wasn’t my biological father I honestly wouldn’t care. he’s my dad, that’s all that matters, and if your ex was a loving father it’s possible that that’s all that will matter to him as well.
People like to imagine that it’s going to bring up resentment, hatred, bad feelings etc but that’s not necessarily the case.
Just go with the flow on this one.