Responses
If he's into psychology get him the book "Thinking, Fast and Slow"
If he's into art get him a sketchbook from a crafts store
If he's a total fucking nerd.. I.. don't know..
If he's into a certain sport maybe get him a shirt with his favorite sports team
If he's a furry then get him a pair of ears
Just find something similar to his biggest interest, if you don't know what he likes then ask him. If this is a "Secret Santa" kind of thing then ask his friends or just observe him like a stalker throughout class.
What is a good christmas present for 15 years old boy? Something not expensive