it may not be as bad as it appears on the surface. If I have read your story correctly, you don't suspect your husband of infidelity to you, but you are obviously upset about not knowing all the details of the situation. You don't give any indication of how old your husband was when this child was conceived by a 17 year old girl. Twenty years ago in many states, 17 years of age was above the age of consent. Relationships which are illegal now were not considered to be more than inappropriate just a few years ago.
I don't mean for this next statement to sound harsh, but I don't know any other way to say it. Whether or not your husband knew he had fathered a child or not, he gave up any paternal rights he might have had in 1999. If the step-dad adopted the boy, he now has a father and may not want to have any contact with someone who has not been in his life. Before you take any action at all, you should consider the effect that it may have on this young man, who may or may not know anything about your husband. As far as the issue of support, if your husband did not believe the child to be his and the mother did not pursue the matter, I'm not sure what you expect that he should have done.
As far as how this will affect your relationship with your husband and your children, please consider counseling. This is obviously causing you great distress. If your husband doesn't want to attend, go by yourself. Sometimes an impartial listener can help you work your way through a problem and find your own answers.
Good luck to you.
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