Non-fiction is your friend. Lots of books have tons of cool pictures and even short chapters on animals, science, countries. Whatever he is interested in.

We also really like the Flat Stanley books. My DS is 4 1/2 and I was pretty impressed with the shortness of the chapters and the quality of the vocabulary.

Maybe a cool book mark might help. That we he won't feel overwhelmed with having to finish the whole book at once.