I second The Littles series and the Boxcar children books. Also Dick King-Smith. Have you tried the Tintin books? You will have to teach how comic books work, but there is a lot of real reading there and the opportunity to learn geography and more.
Rats of Nimh series (maybe read the first aloud and let him continue alone. Sequels are by Conley -- not O'Brian)
The Gammage Cup and sequel. The Moffats and sequels, Ginger Pye, All the Henry Huggins books. the Carolyn Haywood (author) books are probably mostly out of print, but look in the library. Very suitable for younger age but good reader.
E. Nesbit has a lot of description, high vocab., long sentences. Read one aloud to see how it goes before expecting indep. reading.