Adoption of stepchildren, bio mother is deceased

Hi do you know exactly how I go about adopting my son? ME and his dad have been married 23 years and he's 15 years old and his biological mother signed papers in front of a notary giving me all rights to him. From the day he was born he came home with me and has been with us ever since and when he was 2 she passed away. Can you please give me some advice and answers as to how I go about adopting him? Thanks so much for your time God Bless