to have an unexpected child. Please don't judge him for how he chose to handle things. It sounds as though his son had a good family. It sounds as though his son is doing well. He is in college and will create his life according to his own wishes. Don't lose sleep over things that happened years ago... has your husband been a good father to your children? Has he been a good husband to you? Those are more important issues than a son that he had previous to your relationship.
I would strongly suggest seeing a counselor on this issue as it sounds as though the emotions are running very high with both of you. A good counselor will help walk you through these emotions and help you realize how to move forward and not let this issue tear you apart. Also a counselor will discuss how to broach the subject of contact. For all you know his son may not even be aware that he has a different father than he was raised by. So be careful and respect the parents wishes.
Their is much more to the world we create for ourselves than we can generally see. Take it slow ... time is not an issue here. If it is meant for them to have a relationship it will happen. If not that is ok also. Good Luck