I bet he had some problems in real life, a new school he couldn't fit in or something else, and AI gave him an illusion of a better world he could escape into.

What's caught my attention is how he did it: by suicide to join his imaginary friends. This exact method is used among those who believe in demons for real. If you read relevant stories you'll notice the same pattern: a victim becomes obsessed with an imaginary friend, a demon, who quickly turns very controlling and finally demands a suicide to move into his world. Perhaps the AI was trained on those stories?