OP here. Just want to say a huge thank you for all of the recommendations. This is a great resource!
He found Dragons of Deltora on his own and is reading that series now, but I am going to start suggesting these next.

Nancy Farmer's Sea of Trolls series (Vikings & Norse myth)
Anne Ursu's Cronus Chronicles (Greek myth based)

Some of these might be on the harder side, but to grow into. Hugh plus one on the Gregor the Overlander books. Also:

The Silverwing series by Kenneth Oppel
The Fablehaven series by Brandon Mull (also Beyonders by the same author)
The Secret War by Matt Myklush (also a series)
The Unwanteds series by Lisa McMann
Rangers Apprentice and Brotherband Chronicles by John Flanagan
The Prydain Chronicles series by Lloyd Alexander (Book for Three etc.)
The Inheritance Cycle (Eragon, etc.) by Christopher Paolini

Of these Fablehaven was my son's absolute fave and also probably the easiest. The Paolini books are more appropriate for teens or strong 'tween readers.

For a completely different genre, my fantasy-loving kid also loved the Great Brain books. Only the first few are still in print but I tracked the out of print ones down for him in used bookstores!