To be fair he knows it's abuse and isn't really (fully) suffering from Stockholm syndrome. He was just trained as a kid to associate abuse for love, and by the looks of it the sexual nature of this situation (on top of the obsession the r*pist have with him) turned into a sick need for what little bit of acknowledgement he can get.
Basically his parents groomed him
Yeah. It's kinda like how daddy or mommy issues can ruin people's relationships. They taught him abuse is a form of love. He knows its wrong, but its already internalized to the point where he can forgive abuse if it means his r*pist can give him the attention and feelings everyone deprived him of.
This could have been a 10/10 psychological and angsty story where they both have issues at home, wound and heal each other, ennemies to lovers/litc typa shit but noooooo he had to kidnap him and give him Stockholm Syndromwasted potential