My grandfather was born on the wrong side of the blanket. He never knew his birth father. I just recently found his birth father's name in a criminal case. So you can guess why he didn't know. So do I follow the family line of the man who raised him, or the man who was his biological father? Maybe both? I feel like the man that raised him was his dad and made an impression on his life. the other was literally a passer by.
I would research both - the biological line for accuracy's/genetics sake, and the line that raised him for sentimental reasons, to honor/memorialize both.