Well, let me say that no matter what you choose, it will be wrong to someone. My advice is to get in touch with the bio-dad and see if he even has any interest in meeting your son. See if maybe it was a misunderstanding that he lost contact with you. Maybe he got a good job and moved for that and didn't think twice about letting you know due to his immaturity at the time. You just don't know.
As far as your son, I would wonder why is seeking him at this time. Is something going on in his life to make him feel insecure? Is your husband not meeting a need that he has somehow? It is hard to tell sometimes.
I put my daughter in touch with my ex and she got hurt. He pretended to care and it came out as time went by that he was just interested in how he looked to others, not that he really cared about our daughter.
I would protect your son. That is why you need to get the truth from the bio-dad. If he does not respond, you need to tell the truth to your son. Tell him that he is not a nice man and that you truly do not know why is acting the way he is. Make sure you DO NOT put him down to your son because you don't want you son to feel like there is something wrong with him. Ya know?
Oh...it is so sad when men act like that. I would keep pointing out that he is not his Dad, but the man that made him. His Dad is the man standing right beside you. :)
Good luck!