What hobbies or sports is he in?
When my 15 year old was 13, he was (and still is) is into fishing gear, fishing clothes, baseball gear. He also loves character tees of his favorite shows or anime or even blogs (for his bday I got him a 2set t-shirt). He got a new trumpet for his birthday (upgrade from his beginner trumpet). He also loves history, so we've gotten him books on different history topics before. Of course video games are always popular, all of my boys love google play gift cards, so they can get what they want later.