Is there anything in particular he likes? If it's photography you're after, I highly recommened the works of Robert Doisneau. He protrayed France (mostly Paris) pre, during, and post WW2.

Currently, I am reading "Americans in Paris - Life and Death under Nazi Occupation" by Charles Glass; next on my list is "When Paris Went Dark" by Ronald Rosbottom. I live in Paris, but I was born & raised in Germany, so the fact that Hitler left Paris unscathed (unlike Warsaw for example) interests me.

r/Book_Recommendations might also be able to help you out.

If you're looking for picture heavy books, I would recommend you try and find some works from the old Time-Life World War II series. They can be found used for a few dollars, usually. It is a set of 39 books on various aspects of the conflict, and they all are a mixture of chapters on various topics that fit into the larger theme of the work, interspersed with photo essays. To be honest, its been a bit too long for me to quite remember what the expected reading levels of an 11 year old would be, but the works are definitely not academic in tone, and aimed towards a general audience. Possibly a bit dense, but at the very least the photo-essays would be very easily 'digestible' at that age.