It's not the weapons that cause the concern. It's that he's used his vivid imagination to plan out, in detail, taking revenge on his bullies. It could be the best book ever written, and literary genius, but it's an also an insight into his head.
I read Stephen King from about 13, and I lived in the horror section of the library for a while. There is a difference, though, between reading and writing. There is a difference between enjoying someone else's story, and writing a horror story that almost fits your own situation.
It may be an extreme comparison, but it's not unusual for the parents and classmates of those who carry out school shootings in America to say, we thought he was joking, we thought he was just being creative. Infact, those joking conversations, those creative drawings and writings, were planning. They were an insight into where those children's heads were, and how they were processing their situation at that time.
Please don't think that because he seems okay to you, and because he doesn't have access to a nail gun, this isn't a concern.
I mean, I can watch or read all the horror that I like, and I can even write it. If I start writing a graphic story about how I'd like to kill my boss, though, it would be concerning. It's a step away from just enjoying horror films, it's applying the violence from horror to real life.*
*For the record, I don't want to kill my boss!