While he can fight it, it does sound like you would have sufficent basis for a termination of parental rights. This gets filed together with an adoption petition by your current husband, and assuming all is completed successfully, your husband becomes the legal father of your son, and the biological father ceases to have any legal connection with the child. The birth certificate is changed to reflect that.
In order to do this, you need to have an attorney, it is hard to imagine any way you could navigate all of the issues on your own. The Dauphin County Bar Association no doubt has an attorney referral service, and can give you the names of attorneys who specialize in this sort of work.
The adoption act provides for the methods in which a court may terminate a parents rights. There are generally 8 grounds with a best interests section. At the end of the day, the court will look to what is best for the child. You should consult with an attorney. If you cannot locate one, contact the Dauphin County Bar Association lawyer referral service located on Front Street.
If your current husband is willing to adopt your son you will most likely succeed. A petition to terminate parental rights needs to be filed (along with an adoption petition) and you will need to get appropriate service on the father in order to proceed.