I am curious as to why this has occurred now, were there are other issues you have with your husband and you are motivated to take a closer look?
He was in denial over the pregnancy but you were as well. You didn't insist on a paternity test in order to stay in the relationship, you took him at his word. I'm not saying this in a blaming way, I'm saying that there is some shared responsibility for keeping yourselves in the dark.
It sounds as if you have lost some respect for your husband over the way he handled this. And he handled it poorly, no doubt. However he is twice as old now and may not make the same choices today. Try to look at the man he is, not the boy he was.
If your issues are more about the man he is now, by all means get some guidance from a therapist.
I think your children must know about this from you and your husband because there is a strong possibility that they could find out from the other son one day and that would be shocking. Protect them first, above the fear of how they may view your husband once they know the truth. This would be my first priority in couples counseling, I think it's the most important issue.
Good luck to all of you. This is a difficult hurdle but not insurmountable.