An oldie but a goodie, Watership Down
Also he is likely old enough for Ender's Game and it's direct sequel Ender's Shadow. But I suspect Ender in Exile might be a bit much for a coule years though I would have enjoyed it at his age. I would suspect he would not like the other story arc just yet. Those are the "Speaker" arc as well as the "Shadow of" arc which were written for a more mature reader. But in a couple years I bet he would like the whole series quite a lot. The nice part is in the first two the protagonist and most key characters are kids.
http://en.wikipedia.org/wiki/Ender's_Game_(series)
Bugs me to recommend anything by Orson Scott Card because I don't care for his politics but hey, I enjoyed L. Ron Hubbard's SF a lot, he was very underrated as an SF author. Battlefield Earth was a fine novel.
Another set of stories that he might enjoy are an OLD group of anthologies about the Black Widower Club originally created by Isaac Asimov. Basically they are short stories all of which are very classic "locked room mysteries". I loved them as a kid because the puzzles are real and can be solved if you think about them. I don't know of any ebook editions though.
http://en.wikipedia.org/wiki/Black_Widowers
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