I love so many of the books mentioned in this thread!
Any Roald Dahl books, The Lemondade Trick by Scott Corbett (I'm reading that to my kids), the My Father's Dragon series by Ruth Stiles Gannett, and the Mrs. Piggle Wiggle series by Betty MacDonald are all good ones too.
Wow! Thanks for all the great ideas, folks, I really appreciate it.
And I had totally forgotten Treasure Island. I think that'll be a great "end-of-the-school-year" present. His family lives on a rather large piece of land, and he has his "fort" in the woods where he goes to read and eat peanut butter crackers...I'm sort of jealous!