As mentioned above, you’ll find it difficult finding a never-ending source of suitable books.
When I was younger, I spent most of my spare time reading (non-jewish) fiction. Now, looking back, I regret that no-one pointed out to me the world of non-fiction.
There is a far wider ranger of suitable non-fiction books. Fiction is, of course, more interesting for a child, though you may though find ways of convincing him.