I read this book as a kid and I liked it quite a bit though it wasn't necessarily a favorite. I have not seen the movie but the previews have made it look like it is only very loosely based on the book. In fact, the preview felt so unlike the book that I didn't even know what the movie was until it gave me the title. It definitely doesn't have the feel I would have associated with the book. I don't think I'll be watching it unless it comes to Netflix (and then only maybe).

I am like your son in that my philosophy of movie watching is to always read the book first (or not at all). I am still not watching the Game of Thrones series for this reason. The books are not complete.

Most movies that are based on books lose something in the translation but there are movies out there that have done a VERY good job. I still think the Harry Potter movies have done the best job of preserving the story and feel of the books.