Would "War in World History" from the Great Courses work for a 6th grader? When I asked him what parts of history he's interested in, he says he's interested in WWI & WWII, and history of warfare. Obviously that course doesn't spend a lot of time on WWI & WWII, but, I feel like he's probably a bit young to throw something like Utopia and Terror in the 20th Century at him (which otherwise sounds like it'd be great, so if I'm wrong and it would be okay for a 6th grader, let me know). Likewise, I'm not convinced that the separate courses on WWI and WWII would work either because of age. Also, I don't think he really cares *that* much about the two world wars - his first reaction was that he doesn't have a specific time period he's interested in, which is why I'm thinking WiWH might work.
There are also courses about Military Blunders, or about Decisive Battles, but I'm not sure how much context those give for people who are still relatively shaky in their world history knowledge (he's listened to all the SOTW audiobooks, and likes to read and reread all the Horrible Histories books).
For reference, he's completed Thinking about Cybersecurity (his choice, he liked it), and we're partway through How to Become a Superstar Student and An Economic History of the World since 1400.