To keep your child engaged with reading, you should keep a wide variety of books on hand, and be sure to include books on topics that interest him.
In your book collection, keep books that your child currently enjoys so he can read them over and over again. Repeat readings are great – they help kids feel comfortable with the story and begin "reading" it along with you.
Visit the library to choose books together. This will allow you to introduce new material regularly. Picture books are good because they allow you to point out words and help him begin to recognize letters and their associated sounds. Pictures also give clues to the story for young children who are just grappling with language–learning, but if he can follow the plot of a book without pictures, that's wonderful!
The important thing is to go at his pace, but maintain a rich and varied literary environment.