get a lawyer. the young man maybe not be yours biologically but he is yours emotionally and a lot of your relationship with your wife was with him at the center. get counseling. dont over react. you do have a family and all families have made some mistakes... pray
Get a paternity test, then file a refusal of child support because you are not the father. Seek for the biological father to pay. Get a good father's rights attorney.
Get a lawyer, no matter what. Also, even though the child isn't yours, try to handle business without scarring him. Don't let the divorce turn too ugly, for the child's sake. Get it over with as soon as possible. It's worth losing some things in the divorce to get it over with quickly.
The first thing you should do is consult with an attorney and talk to family pshycholigist about how this is going to effect your son, even though you are not the biological father you are the only father he knows. It is imprtant that you do not become vindictive towards your ex.
My advice is to sit down and really think about this. You are the only father figure this little boy knows. Your wife screwed up, but the little boy didn't.