An absolutely terrific, and often underused, resource is your son’s SCHOOL librarian! I worked for several years volunteering in our elementary school library and was amazed at how well the librarian knew what appealed to kids, what was popular, and what wasn’t popular but was still wonderful. She produced lists for the kids such as “If you liked the Harry Potter novels, you may also like…” So get to school and see the librarian or email him or her. Even better, ask if you can volunteer in the school library. They usually do need help. You will learn SO much about what’s out there for kids!
Meanwhile: Yes, the Percy Jackson series by Rick Riordan. Start with the five-book series that begins with “The Lightning Thief.” Riordan has a couple of other series since then (including a new “young Olympians” series and another one based on Egyptian mythology). If your son digs the magic in Harry Potter, he might well like the mythological beasts and gods in the Percy Jackson books. Entertaining!
Try “Eragon” and the “How to Train Your Dragon” books (though the latter may be too young for him–?
If he likes fast-paced mysteries and exotic locations: Try the “39 Clues” books; again, several different series; the “39 Clues” ones are the originals.
Charlie Bone occurred to me too; someone else mentioned it. My daughter only read the first book and wasn’t that into it BUT I know a lot of kids (both boys and girls) enjoyed it. I might wait before suggesting that one to your son; it’s similar in setting to Harry Potter (academy for kids with special powers).
In the non-magical realm, try humor, especially humor revolving around kids his age. Anything by the author Louis Sachar is great - his best known novel is “Holes” and he also has a series of very funny short stories about “Sideways School” that really appeal to kids. Is your son into the environment? If he is, and is ready for some pretty smart novels, he might try Carl Hiassen’s books like “Hoot” and “Flush” in which kids break some rules to help out wildlife and the environment in very funny ways.
My daughter hasn’t read them but I know kids who ate up the “Series of Unfortunate Events” books.
Ask the school librarian to round up a good handful of books she thinks he might like based on the fact he likes Harry Potter.
One other thought: I’ve read a couple of articles about studies showing that boys often prefer nonfiction to fiction in the elementary years. So encourage your son to read nonfiction that interests him as well. What’s he into? Arts, sports, drawing, jokes, history, animals? There are great kids’ books on all the above at the local and school libraries. I saw many boys check out “list books”- things like the Guinness Book of World Records or “Top 25 Football Teams” etc. and that’s still reading, even if we parents wish they’d read novels. Also very popular with both boys and girls were “how to” books - how to draw Pokémon, how to play chess, how to make origami Star Wars figures (for real!), and so on. Even those are still reading. Don’t limit what you suggest to just novels. Have fun!