1) Keep him well stocked in non-fiction. Nothing wrong with reading lots of non-fiction.
2) Look for Chapter Books where each chapter is an individual story. Books like Mrs Piggle Wiggle, Pippy Longstockings, Dr. Doolittle.
3) Read longer chapter books with him. I read to both my kids well past the age they could read to themselves. We usually read at bedtime approx. a chapter a night on a story that he wasn't willing to read to himself.
4) Try magazines. National Geographic Kids, or even the adult version was very popular in this house.
In K & 1st my son would only read non-fiction or more advanced picture books. His decoding skills were excellent and he could read anything. But he didn't have the attention span/intrested in reading chapter books. One thing to keep in mind is that many picture books are a higher lexile level and have a more challenging vocabulary than many early readers.
Don't forget to go to the library and borrow books where you can. When your son reads so quickly it can get expensive very fast to buy all these books.
