Tricky one isn't it. He would have to know about the holocaust to really understand it - then again I know that I read When Hitler Stole Pink Rabbit when I was the same age as your DS and certainly had no knowledge of the holocaust.
The book I felt was aimed at young readers. Bruno is age 8/9 in the book, so a 9 year old could relate to it quite well. It does not go into graphic detail, people just go missing. It is only at the end that there is a bit more mention about it. I think if you had a chat with him about what happens in war time... people die, people aren't nice to each other... then let him decide if he wants to read a book which involves wartime and death.
I'm seeing the film tomorrow... it's been rated 12A for scenes of holocaust threat and horror. BBFC considered that the films content generally was suitable for 8 year old to watch, apart from the ending... which is why it's been rated 12A.
I would say that generally the book will be fine for a mature younger reader. But the last two chapters should be read with you present, should he wish to ask you questions, require assurance that it doesn't happen these days etc.
Yes, read the book first yourself... it's not that long.
Just seen the film. The end scene is the distressing part. As an adult watching it, I could fill in the gaps for the things we weren't being shown. If a child has been finding out about war and the holocaust, gas chambers, concentration camps, then I think the film will help make it all more real. But if a child know's nothing about it... best they don't see the film. I expect this film will go to DVD quite quickly so it can be used in Schools, when they cover the holocaust.
That review is one persons take on it, and I suspect they were an adult reviewing a children's book. The book isn't meant to be historically accurate. I feel it is meant to be away of getting children (KS3 particularly) to find out a little about the holocaust. It creates interest in the subject matter which the children then go on to study in more depth in their KS3 History lessons.