I lost track of who else wanted the spoiler. So I'll just put it in here:
All the girls loves him. Their fetish is to get him to enjoy pain as much as how they want pain to be inflicted on them. So the pain the boy has must be equal to the amount of pain they're receiving. Mostly they're training him to be both the person who can torture them and be torture. They want the equal feeling and love receive by him.
He eventually is allow to leave but he'll become someone that the girls want him to be. He'll be a full-blown pervert. He isn't allowed to leave till I believe volume 4? Or at least from the images it shows that he finally left around there. So more details is far away from now.
So they're training him slowly unlike the others comic where time skipped. So this won't have time skipped, just a day to day life.