Yeah, the best thing about that story was its ambiguity. Not your typical romance fare.
Spoiler (highlight to view)
Well, you remember the police at the scene revealed that his wife hadn't been pregnant with a child at all, but had had a parasitic fungus inside her? She gave birth to at some point around her death, and it grew into that YKK-esque person-shaped mushroom. I'm pretty sure that that is what we're supposed to assume has happened to his mother.
You could draw other conclusions from that, such as perhaps the parasitic fungus really is his child, or anyway comes from inside him. In other words, he is full of and can only create corruption and hideousness.
Anyway, if you want to discuss it further, the best choice would probably be to head on over to the story's forum.