Some good roughly 3rd gradeish series are:
Boxcar children, Hardy Boys, Bobbsey Twins, Time Warp Trio, Geronimo Stilton, Dog/Cat Diaries, Black Stallion series, James Harriot books (animal based), Half-Magic, Moongobble & Me Series, Indian in the Cupboard, Hank the Cowdog, Bunnicula series, A to Z mysteries, Choose your Own Aventures (a few variations differing by reading level), books by Marguirite Henry (mostly horse books), The Littles, Amber Brown, Wayside School, Secrets of Droon, any book by AVI, Magic School Bus Chapter books, books by Ronald Dahl,
other ideas would be:
Biographies (some really good ones on Mark Twain, Helen Keller, Alexander Graham Bell, etc) really have some great authors at about the 3rd grade level.
Good Magazines : Ranger Rick, Ask, Time for Kids, National Geographic for Kids, Calliope, Cricket, are few
Also write a few titles down that he likes and do a ' book-a-like' at scholastic. They will refer you to similar style books!
A good chat with a childrens librarian also is a great place to find new titles that fit 'styles' and/or types of books.