Would be a family line. His son whom he is unaware of from a relationship he had over 20 years ago while he was just making a name for himself as a patch with the pagans. The relationship with the mom was more a matter of convenience, keeping up appearances and expectations within the mc. She wanted more than he was willing to give and they parted ways. Was very short lived but she'd have had photos and things from when they were together.
My thoughts are that the son isn't necessarily trying to track him down but has become a prospect with the mc and found him that way. He could then be involved in trying to find out who got his old man sent away for manslaughter once they get to know each other as it was someone in the pagans. When and how he discloses who he is would be up to the writer. He could keep it to himself himself awhile or drop it like a bomb.
Might even be interesting if he was there to make Frank's life miserable. Maybe he didn't have a great childhood with whoever his Mom wound up with. Maybe he blames his bio Dad for that. Again, up to the writer but I am all for some drama and twisty plots.