Hi there 🙂
You have my utmost respect for the way you have handled this situation...your wife and son are very lucky to have you, but I reckon they know that! 😉
As you are on the birth certificate and married to the mother, in law you have Parental Responsibility. You do have the option of leaving things as they are, the bio father has said he wants nothing to do with your son, but I understand your worry that one day he may change his mind. You could leave things as they are but discuss it with your son when the time is right, so if the other man does turn up in the future it wont be a shock to him.
Without Parental Responsibility the bio father has no rights at the moment, he would have to apply to the court for this, he would probably want DNA proof, and would also have to apply for a contact order. If it were to happen it would be a process that would take months of court hearings, reports from police and Social Services, and your son would be talked to and his wishes and feelings taken into account. As he has never met your son the court would be very cautious before allowing this....Theoretically they could agree to the father having indirect contact to start with, but only if your son expressed a wish for that to happen. Then introduction of this man would be slow and measured, with your sons well being given the only priority.
Its a tough one, the hardest thing being to find the words to tell your son the truth. In my opinion, as the bio father now knows, there is a risk that he may change his mind at some point and try to take it further...you have some tough decisions to make....
Children are very resilient and I'm sure your son feels loved and secure, it might be best to tell him sooner rather than later...but that of course is a decision only you and your wife can make.
It might be a good idea to seek some legal advice, some solicitors offer a free initial consultation or I could ask the Forum moderators
to as the Coram Childrens Legal Centre (CCLC) to come and give you some advice. If you would rather that advice be confidential there is a link to their website and free phone number at the bottom of the page.
I really wish you and your family well, and sincerely hope that you can weave a path through this and find a solution that will cause the least upheaval for you all