I second Twenty and Ten; it is a Sonlight book. I also highly recommend The Young Underground series. It contains eight books which you will probably need to buy used, as I believe it may be out of print; I was not able to find the books new at the time I was assembling the series. I bought all of the books used, but had to wait quite a while before I could find the eighth one at an affordable price. The story revolves around a brother and sister and their friend during the Nazi occupation of Denmark.
Snow Treasure is a FABULOUS book. I read it as a child, and my children and I have listened to the audio book at least twice, and it was the first audio book my children ever listened to - they were 3 and 7, I believe. We rechecked it a couple of years ago, I believe they were 9 and 13. It is based on a true story of how the children in a Norwegian village smuggled a fortune in gold out of the country, right under the noses of the Nazi occupiers. It is very thrilling. This is probably my number one pick of all the WWII books we have read. The Young Underground series probably comes next.
Number the Stars is also an excellent book, but it does have some disturbing parts in it. The one that comes to mind is the part where you find out what happened to the older sister - who is dead at the beginning of the book. There are some events that are very frightening to the characters as they face danger. I believe my children were probably around 6 and 10 when they first listened to that one as an audio book, but it could have been a year later.
The Great Escape is a marvelous book, but as it is an adult book it is probably better suited to a read-aloud, so you can pick and choose the parts you read. My children were 4 and 8 when I read it, so that is what I did.
If you can get hold of an old How and Why book, that will be a wonderful resource. I have both the WWI and WWII books; I bought them a couple of years ago. Bookfinder.com is an excellent resource for locating old books.
We also read Nights of Danger, a great book about an American boy living in France because one of his parents was French - his mother, I believe. He becomes involved in the Resistance. It is very exciting, and both my children really enjoyed it.
Taffy of Torpedo Junction is an exciting book. It is based around the submarine that was on the Carolina coast during WWII. I was racking my brain trying to figure out the title, because when I read the book aloud to my children, I used my daughter's name in place of Taffy's, and our mule's name in place of her horse's name. But I managed to find the title just now. :-)
I would think that I would be able to remember more of the books we have read. As I said, we have read about WWII ever since our children were little. We've just read so many books that I can't remember them all. But these are the ones that stood out in our minds and came right to mind.
Later, when your son is older, you might want to read Where the Ground Meets the Sky. The story is set around the Manhattan Project. There are some disturbing things in the book, such as when a cat is exposed to radiation and instead of being put down is left to suffer so they can "study" her. The cat's owner, a teenage girl, puts the cat out of her misery out of love. The main character's mother ends up in an institution because she just cannot handle the knowledge of what her scientist husband is doing. This is a book that you will not soon forget after reading it, but your son may need to be older before he is ready for it. I believe my daughter was 8 or 9 when I read it aloud, and had I pre-read it before beginning it aloud I probably would have waited. You know your child, and what he can and cannot handle, but if you choose this book you might want to pre-read it first.
Goodreads has put together a list of children's books about WWII. It is at this link.http://www.goodreads.com/list/
You have to be careful though, because many of the books are for older children. I do NOT recommend The Boy in the Striped Pajamas. The end is VERY disturbing. I checked it out from the library and pre-read it - which I do as much as possible before giving books to my children - and when I got to the end I just put it back in the bag and returned it to the library. When your son is older he can probably read this book, but not at the age of eight.
I hope this is helpful to you. I wish I could remember more of the books we have read, but there have been so many I just can't remember. I wish I had been better about writing down the titles of all the books we have read over the years. That seems like a good "resolution" for this year. I'd better see if I can remember the books my daughter and I have read so far this month! We've been burning up our library cards!
I hope this great review of WWII books was a great help to you.