How about one of the QI books, or the pointless one. My nephew who is 11 loves the QI ones and they are certainly full of fun facts.
Depending on what sort of facts he likes he might also be interested in Schotts Miscellany.
For general non fiction I'd suggest Bill Bryson. All his books are engaging and accessible but full of all sorts of weird facts. You could dive in at the deep end with a brief history of nearly everything if the length of the book wouldn't put him off (at his age I'd've seen it as a challenge, but then I am strange and read War and Peace at 12 because a teacher suggested that I read it as a joke). Or for something shorter I like mother tongue (but beware there is a chapter on swearing).