In the United States, there are clubs that "re-enact" Civil War battles. These are marvelous spectacles. They show the tools that were used, and that the soldiers involved were ordinary people. If there are any World War II "re-enactor" clubs near you, you could bring your son to an event, and introduce him to some of the members.
You are wise to truthfully answer his questions, and to not go out of your way to discuss topics that you do not think he is ready for.
When you think he is ready, you might want to introduce him to some good books on the topic:
- The Diary of Anne Frank. I have not read this book, but I understand that many elementary schools have children read it.
- Caged Dragons: An American P.O.W. in WWII Japan by Robert Haney. This book shows why so few war veterans are able to describe what they went through during the war. It is very clear. It tells just what Bob Haney saw, in very understated (but powerful) language.
- The Bridge at Remagen by Ken Hechler. This book emphasizes daring, heroism, and the roles of chance and logistics in the outcome of a single event on a single day.
- The Encyclopædia Britannica article on the "World Wars" from the A-Anstey editions (circa 1970), plus the world and historical maps in the appendices. This article is book-length. It is comprehensive, but does not provide a feeling for what it was like to experience the war.
- The Road to Serfdom by Friedrich Hayek. This book provides economic background, and explains how people could become so dependent on governments that were willing to (and/or felt forced to) grind them to dust.
Belgium is within driving distance of where Anne Frank lived, and of Remagen. If your son wants to visit these places, you might be able to arrange it. Also, some people who lived through World War II are still alive; you might be able to introduce your son to some of them. Sadly, Bob Haney has died since this question was posted.
"When you think he's ready" probably means "over the course of a few years". Your son is now 11 or 12 years old, so most of these books are now age-appropriate. The movie version of The Bridge at Remagen has a PG rating. If Caged Dragons were made into a movie, it would probably also get a PG or PG-13 rating.
Caged Dragons has a lot of similar themes to The Hobbit. If your son has read and understood The Hobbit in English, then he is probably ready for Caged Dragons. Based on how he reacts to Caged Dragons, you can decide whether he is ready for The Bridge at Remagen and the "World Wars" article. Based on how he reacts to the "World Wars" article, you can decide whether he is ready for The Road to Serfdom.