I don't have primary aged children, but I am an avid reader, and always have been. I would be asking, what sort of books does he enjoy reading? Because reading should be fun, and from that the learning comes. The more you read, the more you learn (and I don't just mean from book content) so I would be encouraging him to read books that he finds enjoyable. If he likes a challenge, then that's great too. Do you belong to a library? That's a great, cheap way of letting him go for it when choosing his own reading.