I have to chime in with @keobooks and @janbb and say that in my opinion it is not appropriate for a young teen.
My fourteen year old is a very precocious reader (right now he is reading The Good War by Studs Terkel), and I steadfastly refuse to let him read the Dragon Tattoo books until he is older. The book has lots of very disturbing sex and violence that I think most 12 year-old’s would find upsetting. I have always let him read whatever he is interested in, but I have just had to say no to that series. Come to think of it I might discourage Don Winslow books at this point, but I would let him read those before the Dragon Tattoo books. I wouldn’t even consider letting my 13 year-old read them.
There are so many awesome young adult series out, it shouldn’t be hard to find some great books that are more age appropriate. Has he tried The Maze Runner Series or maybe some John Green? Totally different genres and very compelling reads with more age appropriate sex and violence.