Running out of books to recommend for your son-- that's a good problem to have!
Any of Heinlein's young adult novels are likely to please. Heinlein is very aware of history and references it in his works frequently. Tamora Pierce is also an excellent writer with a strong knowledge of history which she uses to help the realism of her fantasy books. Both of these authors are accessible, but don't "talk down" to their readers. Good choices for smart kids.
You may wish to check out Mercedes Lackey or Anne McCaffrey yourself before recommending their books, as both authors tend to include at least some sexual content (in some cases rather a lot-- Lackey's characters, in particular, seem to be prone to being raped). If some sexual content doesn't bother you, and you think your son would be interested in exploring other parts of history, Jessica Amanda Salmonson's
Tomoe Gozen series or Barry Hughes'
Bridge of Birds books (set in semi-historical Japan and China, respectively) are good reads. Or James Clavell's
Shogun, for that matter. And someone who really enjoys history is likely to enjoy Tolkein, even though his "history" was invented. I would also suggest fantasy authors Garth Nix and Shannon Hale, both of whom incorporate historical and folktale content in their works. Jonathan Stroud has already been mentioned, and I second that recommendation -- that would be a sort of alternate history. Orson Scott Card's "Alvin the Maker" series falls into the same sort of category.
Getting closer to your original question, I'd recommend Gillian Bradshaw's books set in the Byzantine period, particularly
The Beacon at Alexandria and
Imperial Purple.