The anime features him only having a son but how did that come to happen? Him being a superhuman and all, he probably wouldn't settle down for a normal girl, or would he? What is the story there? Did he ever get married?
As far as I can tell from reading the wikia for the show, they have no mentioned the mother yet.
This is less of an answer and more of a "there is no answer" type thing.
Although I'm all for the idea of him having cloned himself into his son. That opens some very frightening possibilities and some interesting conclusions. Does he have more clones? If his son-clone turned out like that, then could the father actually be a good person that was changed by bad circumstances? I'd watch that.