A big seconding to Ender's Game and all of the related books... honestly, I find the stories about Bean to be more interesting than the ones about Ender.
Because he likes Pratchett and Adams, he might very much like Carl Hiassen... different genre, but very similar humor. He's primarily known for the piles of stuff he has written with strongly adult themes, but he's now doing some for the young/teen reader group. Start with Hoot and see how that catches on.
Mark Twain is another great source of that sort of humor, with little worry about adult themes.
Some fantasy I was reading in my middle school years that I'm reasonably sure is free of adult themes:
The Belgariad/Mallorean - David Eddings
The Sword of Shannara (there's a related Netflix series my daughter and I are currently enjoying) - Terry Brooks
Riftwar Series (enormous, begins with Magician: Apprentice) - Raymond Feist
Guardians of the Flame series - Joel Rosenberg
