Cons of the book:
13 year old girl uses crass language throughout
9 year old boy disobeys and mom "pretended not to notice" (this theme is carried throughout the book)
Probably when he's old enough to deal with the book, it will be beneath him. The writing style is geared toward 9 year olds.
Cons of the movie:
Violent goblins and ogre
Minor language (less than the book, I think!)
I think by the time he's old enough to watch the movie, he will still be interested in it. I enjoyed the movie. :-) But I probably won't let him watch the movie until he's 13 or so. I'm developing my own rating system of movies, and it's very simple. Whatever Hollywood has rated a movie, I just bump it up one notch. Makes it easier to decide if he can watch a movie I haven't seen yet.