I just read the first book in a series of 12 from Simon Hawke.  It was called, The Ivanhoe Gambit.  This could easily be better categorized as "fiction" than "historical fiction" but there are enough elements of history for your kid to massage the rules on this one.

Here is the description on the back cover:

Sometimes Sergeant Major Lucas Priest wondered why he had enlisted in the U.S. Army Temporal Corps. Fighting in the Time Wars wasn't the easiest way to make a living. Sailing with Lord Nelson, battling Custer at Little Big Horn, and a year of pillaging with Attila and his Huns had left him ready for a vacation.

But a demented scheme to impersonate the King of England was threatening to change the course of history. Two army teams had already failed to intercept the madman. So Lucas and his men must clock back to the twentieth century to try and prevent an irreversible split in time.

But when Lucas discovers whom they were to impersonate, he almost wishes that Hannibal had killed him back in Carthage. How were Ivanhoe, Robin Hood, and a band of merry outlaws supposed to protect the future of the world?

Link to description

ETA: This particular story takes place in the 1100's.  FYI...There is some violence and some cuss words throughout the book.

Rifles for Watie - Harold Kieth
story of a soldier in the civil war, good read, lowest reading level of the books mentioned here


Alas Babylon - Pat Frank
small town in florida in the 50's during a hypothetical war with the soviet union. i guess it's too recent to work though...


Mr Midshipman Hornblower - CS Forrester
Is the start of a series that chronicles the naval career of an officer in the Royal Navy. Very good read, especially if you like the golden age of sail.


1632 - Eric Flint
Small modern town in west virginia transported back through time and space to prussia in the year 1632

Alas Babylon is not set before 1941, but it is a great novel.

What about McCarthy's Blood Meridian?

"Killer Angels", the story of the Battle of Gettysburg.  It's on my all time top 10 list of great books.  Author is Shaara
Killer Angels

"Patrick: Son if Ireland", Author is Lawhead
Patrick

Guns of the South by Turtledove.

WARNING OF WAR  by James Brady    It is about the North China Marines; the garrison stationed in China pre-WWII .  
FWIW, the date restriction might be pushing it a bit, but the bulk of the storyline is pre-war and the week of Pearl Harbor.   VERY good book.

or ...

The AFGHAN CAMPAIGN   by Steven Pressfield.  He's a master at historical fiction. This chronicles a Roman Legion in ancient Afghanistan as an "occupying" force.
He can draw his own parallels.  Great book.

or ...

MISTRESS of the ART of DEATH    by Ariana Franklin.   A fascinating story about England in King Henry's reign. Main character is a young woman who is attempting to honor the new "art" of death (coroner) not get taken for a witch, and solve a series of murders.
FWIW... Franklin is as gracious a person as I've ever spoken with. She kindly gave me permission to adapt a scene as an audition piece for my son.  She has also sold the movie rights to the BBC and ... if there is a brain left in England ... it will make a hugely entertaining movie.   Plenty of action and intrigue.

Stay safe