How old is your son? 10? It's a very popular teenage fiction series in US/UK. The twilight series are the top 10 bestseller on Amazon (children category) these 2 years. My son doesn't show any interest in these kind of fictions (love story). My daughter picked up the book, scanned few pages and put it down. Probably it's too thick for her to complete it.
The movie sticked close to the book. Since each of the novel consists of 500+ pages, it's impossible to include every single scene in the movie. The director has to re-arrange and modify the story in the movie to suit the taste of the general audience.
There are some love / sex scenes in the story. I'm not sure how you'll classify these contents. Usually Chinese are quite conservative and regarded it as improper content for children but it's not a big deal for the westerner.
Probably, you can read the novel and share your thought with your son. Actually, there is a fan site "Twilightmoms" in US which is a group of parents obsessed with twilight after reading Stepheine Meyer's book! Don't you want to be the next one!