Definitely the older books, as mentioned above.

You might try The Phantom Tollbooth -- my son loved that one around age 4.

Also the Beverly Cleary books, such as The Mouse and the Motorcycle, Henry and Ribsy, the Ramona and Beezus books (I think they just made a movie of one of those?).

There is a great poetry book called A Pizza the Size of the Sun that he might like, and another called Science Verse, I forget the authors.