Does he enjoy being read to?
I would think that at this stage if you can inspire a love of reading, by reading out loud to him, then you can begin to introduce so many other topics. And if you are reading to him, you don't have to worry about too easy or too hard, because you are right there to wade through it with him.
I guess it would help to know what he does like to do..but without knowing, I would just focus a lot on reading aloud.