Lemoncello’s library series is maybe a bit younger but fun.
National geographic has an adventure series for that age now.
Prydain series
Sword in the stone and once and future king
Book scavenger series

Beyonders and Enders Game

My 6th grader has been tearing through Keeper of Lost Cities series.

Has he tried the Alex Rider books by Horowitz?

DS just started the Belgariad, it's classic fantasy but suitable for younger readers. The Modesitt books are also a good suggestion.

Spy school series

Second the Rangers Apprentice/John Flanagan books

First to preface - I have three kids - two girls and 9th grade boy.

Unwanteds series. All liked.
All Creatures Great & Small and the rest of the James Herriot books.
Nyxia triology - all loved. Said best books they read this year. Might be a wee intense...youngest is 7th grade so should be fine.
Genius File books - younger but all like
Hiccup the Viking series...something like that...13 books - upper elementary but mine still reread.

It sounds like he is a strong reader. I would suggest Modesitt’s Magic of Recluse series. My kids like them a great deal. From a parent view: No sex though the main characters do have love interests - they do not play a huge role in any of the books. Food probably has the same level of importance. No graphic violence either. Lots of battles and fights but no gory details. The book titled The Magic of Recluse is the first one, but then you can skip around a bit if he liked it. The Magic Engineer and The Colors of White are good. My boys like Modesitt and Sandersen. God forbid they (and I) read The Wheel of Time series as well. No parental concern issues with that series either though certainly repetitive. It might be a required right of passage if he is going to read the alternative world sci-fi stuff.

I also push to good readers the Hornblower series. Historical fiction. Again no sex and the frequent battles do not emphasize gory details though enough there to keep a youngish reader interested. Start with Midshipman Hornblower and then they run in order by time (throughly oddly they were not written in sequence with the character’s age). If he likes it then as he gets older he can get into the Aubrey/Maturin books which also are pg-13 in content but much more dense and historically “accurate”.

You also might try some non-fiction. The McCoullagh book on The Wright Brothers is great and I can say that I for one did not know much at all beyond the basics. (And I’m not a big McCoullagh fan - his Truman book was painful.) Also, Stephen Ambrose’s WWII books are good reading. He could read Band of Brothers as a good intro.

Endling. The first 2 books are out and the 3rd is due to be released in March 2021.

Redwall is great.

Storm Runner series. There are 2, another coming soon.

The Redwall series, the Septimus Heap series, 39 Clues series, the Charlie Bone series

My kid liked the Ranger's Apprentice books, and the Inkheart trilogy.

My son is looking for book recommendations. He’s 11 and starting 6th grade. He has read and liked the Eragon series, Gregor the Overlander series, Wings of Fire series, Maze Runner series, & Land of Stories, Nathan Hale’s Hazardous Tales series, all Rick Riordan books, & Harry Potter, if that helps. I am reading him LOTR. He says he likes action and adventure books and series that take longer to read, over single books. He says he knows other people like the Warrior series but for some reason he didn’t really like that one. I’m about to give him the Hunger Games to read next.Thanks for any other ideas!