I've recommended them here before: I really like Naomi Novik's Temeraire series for boys that age who are fluent readers. It is fiction, the genre of it is "fantastic" alternative history: the premise is that there was an air force during the Napoleonic Wars, and that it consisted of trained dragons and their riders. What I find fun about this series is that she has taken real events and overlaid this layer of magic over them.
As for military history, I majored in it, and so I'd have a question for you: how does he feel about politics (not about taking a side, just the influence of politics on the things that nations do.) If he has an appreciation for that, then wading right into major military histories such as those by John Toland should be fine, but if what he really wants is battlefield narrative, then a good place to start might be the books of Peter Fitzsimons. Fitzsimons is probably Australia's best-selling non-fiction author, and his specialty is battle narrative and military biography. He writes in a very populist style, though he has been accused of embellishing a bit at times in order to improve the narrative flow. (Things like describing the look on a General's face when he said something that he is really known to have said -- even though no one present at the time commented on that aspect.) He is Australian, of course, so a lot of his books focus on Australian history, but it's an interesting country, and as part of the British Commonwealth, sent soldiers to fight in all of the major wars of the modern era. Fitzsimons is a bit of a character; he's a former professional rugby player and is a well-known sportswriter, too.
As for American or British popular military history, there is no one quite as accessible as Fitzsimons, but I'd recommend the works of John Keegan, Stephen Ambrose and perhaps Cornelius Ryan. Bernard Cornwell's histories are also good if he wants to go back to the pre-mechanized era. (Stick to the histories, though, there are some rather adult themes in the novels, which much as I love them (especially the Sharpe series) are probably not appropriate for a 10yo boy.))
It's good to see recommendations for books that I had looked at but wasn't sure about.
