Family Law - My rights as non-biological father;?

My former spouse is recently deceased. We have always been the best of friends. Up until she died a month ago, she had my last name along with my biological 19 year old son. She subsequently had a 5 year old son, not by me, but the child also has my last name and I am the Father listed on the original birth certificate. Again, we were the best of friends even after our divorce. The biological father didn't want her to give birth to the child. Subsequently she took him to court for child support, initially received but since had long stopped receiving. I have cared for the child since birth to include birth, hospital visits, billing summer vacations, etc. Even now my son (5 year old) is in my care in Atlanta. He has always thought and still thinks I am his father. There have been attempts by an aggressive female friend of my former spouse to extract my son from my home. She has taken steps to obtain a NEW birth certificate without signatures but has the INCARCERATED biological fathers name on it. The "friend" traveled from Florida, boldly visited my house accompanied by the local police. She presented me with an affidavit from the biological father naming her the temporary custodian/guardian. I refused to give him to them and they left. The biological father and I had a good rapport before he was imprisoned and is not aware that the child is in my care. I am writing him a letter to see if he would sign over guardianship/rights? If he doesn't, what can I do to ensure my son stays in my life permanently?