It always feels like the mob npc parents aren't really his parents, and that it's most likely Delkira + a human woman who are.
I have been wondering that for an extremely long time now. There is no reason why his parents would be that shitty to him...sure some parents are absolutely terrible to their children, but this is just a whole other level of that. Sure, it may have been played up for laughs occasionally, but on the whole they are terrible and are presented as such. These parents of his are not his real parents and may have just been pushed into "raising" him for Delkira and his real mother, but really they wanted NOTHING to do with him.
The other option is the sadder one: he is an orphan and has no parents at all. His parents (whether he is Delkira's son or not) are just not in the picture. These 2 individuals are either people who just happened to pick him up one day and thought he COULD be useful to them OR they are just a figment of his imagination as his mind is trying to justify his shitty treatment by putting it all on parents that don't actually exist.
No matter what, he has trauma from living in the human world because he was treated terribly and lives a great and fulfilling life in the demon world. He would not want to give up that life no matter what, even if he knows there is a possibility of being forcibly returned to the human world.