He mostly reads a lot of 'younger' books like Diary of a Wimpy Kid and Dog Man. That's fine, but I think it would be good to read slightly more challenging books too.
He liked The Explorer, but found it too long. He's quite a slow reader. He's recently quite enjoyed War Horse. Before that he liked The Giver. He also loves graphic novels.
Any recommendations for not-too-long, age-appropriate books that might interest a 14 yo?