I don't know if you are looking for comics (sort of assuming so as this is on the Maus Forum), but Runaways: Pride and Joy v. 1 (Runaways Digest (Quality Paper)) is a young teenager comic. I haven't read it personally, but I trust Brian K. Vaughn as a writer and have heard good things about this series. If he's use to something a little bit older Batman: Year One - Deluxe Edition is more adult, but that's if he's allowed this type of thing. It's probably a little stronger than the current Batman films, contains mentions of prostitution and is generally 'dark'. Failing that, Maus itself I remember being excellent, but I haven't read it in about eight years. So I would have been about your nephew's age or younger.
Fiction wise, at that age I was given three books. Orwell's Animal Farm and 1984, and Burgess' A Clockwork Orange, the first two I didn't enjoy until I was a little older. I really enjoyed A Clockwork Orange, but obviously that's not necessarily an appropriate book for a 13 year old, might need a conversation with his parents on that one.
Note: I was certainly not an avid reader, before that I'd pretty much only read the Narnia books of my own accord, and stacks of comics of the sci-fi, superhero genre.