You might want to start off with something that is fun - I'd suggest the Captain Underpants books by Dave Pilkey, or maybe something by the 2 Steves
http://www.fantasticf...w-and-steve-skidmore/
or a wonderful book called Fattypuffs and Thinifers by Andre Maurois.
Also, check if your library has a section for emergent readers. These are books for older readers with literacy problems - so the writing itself is pretty simple, but the content is aimed at older readers rather than them reading nursery type books. It'll give him a sense of satisfaction that he has completed a book on his own without making him feel babyish.
While the famous 5 and Enid Blyton have their place, very few kids are actually interested in them these days - it tends to be parents borrowing them thinking 'oh, I liked this when I was young' but there isn't much in them that today's kids can relate to.