Go to the school library and ask the librarian for suggestions. Do the same at the local library (ask for the children's section librarian). It will make their day!
Charlie Bone has more than three books - at least five, I think. My daughter liked the first one OK but after that found it kind of a paler imitation of Harry Potter ("special" kids at a special school for their powers, etc.) But it's another option.
I have checked out books for classes at our elementary school for over two years now, and the Eragon series is popular though I can't vouch for it as we haven't read it. Also popular with many kids is the Warriors series (the latter is about cats who talk and battle; characters do die; and it's VERY "samey" book to book but some kids love it).
If he doesn't like historical fiction yet, try him on historical fact -- many kids eat up nonfiction books!
I totally second the Percy Jackson series. Don't confuse Rick Riordan's several series!
First you want the five Percy Jackson and the Olympians books.
Follow with the two new series: One is called Heroes of Olympus and has two books so far, the other is the Kane Chronicles and also has two books out.
The 39 Clues series will engage him but don't buy them; there are lots of books, mostly in hardback, and they'll add up; most school libraries and public libraries have them.
He might like anything by Riordan or Gordon Korman. If he would be interested in very good "school stories" he has great options: Louis Sachar's "Tales from Sideways School" series, which is short stories that are very funny and clever; and the short novels of Andrew Clements (his best known is "Frindle") which are remarkable novels about realistic kids in realistic school situations. I suggest you read Clements yourself!