yeah I read the raw and it seems like he was adopted by the white hair dude when he was already a teenager
That is correct but you wouldn't have him saying father to that person. Stepfather alludes to the fact that they are not blood related.
Agree. It would be ok if they left it unstranslated, like a Kor equivalent of Yifu (sworn father), with a note.
“Stepfather” is fine. Since it shows that he is not his biological father. Adoptive father/stepfather is fine.
I believe there is an mistranslation, he is not his stepfather, but more like adoptive father.