uhm it's more like, as a child he didn't have the right grasp about pleasure and didn't have an adult to walk him through the concept of sex and pleasure (which is expected since not a lot of parents ever talk about sex and all the context to it and considers it taboo-- personally i think this is why children become even more prone to sexual abuse, molestation or grooming)
the neighbor exposes him to masturbation and sexual activities (dare i say groomed jihyun to an extent) and manipulated jihyun to believe that he has a mental illness (i'm not sure if that's really the case or jihyun actually does have said mental illness instead of the neighbor just feeding him lies for his sickening intentions)
i hope i worded that right or that it makes sense

Uhm, it's a bit confusing
He seeked pleasure since he was a child, telled it to that neighboor that abused him and developed this mental illnes that makes you addicted to mast*rbation?