The secret series by pseudonymous bosch

Ranger’s apprentice by john flanagan is a bit different but also good

Keeper of the lost cities

DS isn't into Harry Potter or Percy Jackson, but LOVES Stuart Gibbs. Other favorites:
All the other Stuart Gibbs series
James Ponti Framed series
Treasure Hunters (James Patterson/Chris Grabenstein)
Carl Hiassin books (not a series but he has lots of them)

Gordon Korman books have been suggested but has not read those yet.

A Series of Unfortunate Events
Theodore Boone series by John Grisham

+1 Wings of Fire
Endling (by Katherine Applegate)

My son went from HP to Percy Jackson to Hunger Games (if your DS is ready for that). He’s now reading Spy School, but wants to do City Spies next.

39 Clues (series - LOTS of books)

The Land of Stories

Mr Lemonchello’s

The Warriors

Treasure Hunters

Any of the other Rick Riordan series: The Trials of Apollo, Magnus Chase, The Heroes of Olympus

Redwall

Stuart Gibbs and Rick Riordan both have many other book series, my kid loves them all

Wings of Fire

Keeper of the Lost Cities

Wings of fire

Keeper of the Lost Cities

The Mistborn series

The Way of Kings series

The last two are quite a step up in complexity and themes but I'm not sure how old your kid is.

Hatchet is part of a series but my kids didn't read books beyond Hatchet so I'm not sure how good they are.

City Spies by James Ponti

Three Investigators
Dark is Rising
Narnia
Animorphs

If you can get him hooked on Hardy Boys, that would keep him busy for quite a while.

Some of these book can get intense (but so does Harry Potter) and I don’t know how old your son is or what content might concern you. I encourage you to research further if it might be an issue.

My son only likes to read book series. He loved Harry Potter, Percy Jackson, and the Spy School series. What other book series have your kids enjoyed that are similar? I’m looking for Christmas present ideas.