Aside from seconding the suggestions for Neil Gaiman and Terry Pratchett, I'm gonna suggest the Callahan series by Spider Robinson. It's about a bar in upstate New York that has all kinds of patrons, from aliens to time travelers. Even a talking dog and a vampire. There's also lots of puns. Like everywhere. Most of the chapter and short story titles are puns.

I'd go with Neil Gaiman (some of his stuff is mature, but a lot can be read by younger kids also) and Terry Pratchett (who is for everybody, regardless of age).

And maybe try some Gentlemen's Bastards (Scott Lynch), A Song of Ice and Fire (GRRM) or The Queen's Thief (Megan Whalen Turner). They're not suitable for a younger audience (except for the first Queen's Thief book) but they have so much politics and intrigues that he'll probably get bored long before he gets to the traumatizing bits.

(Wow, that's a lot of parentheses.)

and still have on my shelves

Well, let's begin with the Young Wizards series by Diane Duane. I actually got into this series in my late teens/early 20s, but it's meant for younger readers, and I reckon a precocious eleven-year-old would get a kick out of it. In this universe, wizards are given their magic by the Powers That Be in order to fight entropy and its originator, the Lone Power. Touches on some serious themes, like death, but in a very positive, empowering way, IMO.

Ooh, there's the Dark Is Rising series by Susan Cooper. Arthurian legend meets Celtic myth meets... okay, I don't know what it meets. Pick a series where plucky kids save the day.

Anything by Madeleine L'Engle is good, particularly the A Wrinkle in Time series. It's sort of science fiction, sort of fantasy, and the last book sort of gets into religious myth. It's quite a ride.

Also, you can't go wrong with classics like Watership Down by Richard Adams, The Last Unicorn by Peter S. Beagle, and Black Beauty by Anna Sewell.

Speaking of horses, if you or he are into them, try the works of Marguerite Henry, particularly King of the Wind and Misty of Chincoteague. Oh, and let's not leave out The Black Stallion by Walter Farley.

I hope that gives you something to work with. {= )

~Neshomeh

and I'm going to assume you've already gone for Harry Potter

...can I recommend The Divide Trilogy by Elizabeth Kay? It's about a thirteen year old boy who, while on vacation with his parents in Costa Rica, faints—right over a place called The Divide. When he wakes up, he's in another world where magical creatures are real and humans are the myths. (And the magical creatures all have different names for reasons that are later explained in the books, like tangle-child=elf, japegrin=pixie, stuff like that.)

I will warn that the last book is a bit of a Happy Downer Ending, but I loved this series when I was a kid and can't recommend it enough because it deserves more attention.

It's not entirely suitable for the 1-11 demo, but it is suitable for making sure they go to sleep at night. One good whack and they'll be asleep till morning. Never did me any harm, except occasionally when everything goes grey and I wake up in A&E again. =]

...Started as bed time stories Uncle Rick told his son at night, maybe that could work.

Well, John Flanagan's writings are on the higher end of that age spectrum (actually they're written for middle schoolers, I believe) - but I blame that on the vocabulary. Try out Ranger's Apprentice and Brotherband Chronicles, see if you like them. I certainly do.

If that's not quite your speed, Kathryn Lasky's Guardians of Ga'Hoole, Wolves of the Beyond, and Horses of the Dawn series are also quite good.