Free e-books for a young boy that shouldn't be too hard to read:
Conan books: The whole list is here:
Robert E. Howard Conan stories
Two very good short stories to start him on are:
The Tower of the Elephant
Red Nails
Moonfleet by John Meade Falkner. Great story about smugglers. Kinda old and written in a faux 18th Century style, but I don't think it's that hard to read:
Moonfleet
Edgar Rice Burroghs stuff:
John Carter Warlord of Mars (7 books in 1)
The Land that Time Forgot (Trilogy)
Complete Sherlock Holmes
As I think of more, I'll post them.