Treasure Island, Robert Louis Stevenson.
Alice in Wonderland iff still young enough not to think girls are gross. Otherwise you must sneak up on him with the poetry, then introduce the book. (I've had success with reciting 'Jabberwocky' and 'The Walrus and the Carpenter.')
Learn 'If,' by Kipling, by heart. Then teach it slowly and patiently.
As an aside, quite a lot of Kipling is truly excellently boy friendly.
The Proverbs of Solomon, et al. Found in any Bible or Talmud. Bears repeated reading, as most eastern 'wisdom' literature does (The Book of Five Rings, The Art of War, etc.).
While we're at, why not the Narnia books? I loved them, and I'd no idea the Lion was apparently Jesus until I was nine or so.
But by far do not DO NOT forget Michel Ende's marvelous 'The Neverending Story.'
If you're only familiar with it from a movie, you've missed the best part, when the boy 'Bastian must take up arms and become the adventurer he has read about and loved.