Your son may have said something like wishing he didn't have to be with those other kids all the time, or be bugged by them.
Your husband correctly inferred that, if he had not left his family for the OW, that your son wouldn't be stuck having to deal with these other kids and be bugged by them and put in this unwelcome situation. At least he acknowledged this out loud to your son - that his choices as a father had done this to him and put him in this situation with Other Family - because it is the truth.
The "I'm still your father no matter what" stuff can be said in military style as an order along the line of who cares what you think, you're my son and just have to take it. Or it's possible this can be said, meaning as comfort that his feelings for your son haven't changed even though he has a new family to raise and other kids in the house, that no matter what son does or how much son might dislike him or situation, 'I'll still be your father" and that he loves him and will be there for him no matter what, and the life with OW and her kids is what it is, because that decision's done.
Tone is everything.