Jane, has he read any Rosemary Sutcliff? Her books are really ideal for gifted-reader history buffs--we've read several here, and they are terrific. Also excellent for children's historical fiction are Geoffrey Trease and Cynthia Harnett, and we've enjoyed, too, historicals by Alison Uttley, Barbara Leonie Picard and Penelope Farmer. He might really like Kevin Crossley-Holland's Arthur series--very interesting for those who enjoy both fantasy and history.
There are also lots of terrific books by Farley Mowat that would be of interest to a history lover, some of which are very funny as well.
Not fiction, but I wonder if he'd like Thor Heyerdahl's Kon-Tiki Expedition book (forget the exact title--Voyage of the Raft Kon-Tiki, or something?) and Tim Severin's Brendan Voyage--both quite gripping. In re: WWII, I read William Stephenson's Man Called Intrepid at about 9 and loved it--also not fiction, but maybe interesting for him.
Anyway, hope that helps a little....
peace
minnie
