Um, why are people still not reading the story...? Do you think everything is 100% not drawn from other ideas? In your world everyone would be accused of plagiarism, sorry to burst your bubble. His idea WAS inspired by him not copying him 100%. He said it HIMSELF... mmkay?
Drawing inspiration is not the same as taking a source and just changing a few words here and there. Just because it doesn't 100% follow the source exactly does not mean that it isn't plagiarism. That being said, there are millions of people out there that will have the same ideas, and there will always be an art piece that resembles your own. So it is definitely impossible to be 100% original. The issue isn't that he isn't original though, it's that he copies without regard for those that toiled away thinking about what to create and how to execute it. It's just... disrespectful.
Ahhhhh this protagonist man. I pity him for having trouble coming up with ides, we've all been there but at the same time its like, you cant just flat out copy people's work like that T^T. I hope somehow being around Yohan helps Semin to actually get some real inspiration!