My own 11 year old is still asleep, but as the primary book purchaser (LOL) I know his favorite series (that I think may be fantasy but am not certain) are:
* Anything by Brandon Mull (
Fablehaven,
The Beyonders,
The Candy Shop War, one of the
Spirit Animals books)
* Anything by Rick Riordan (
Percy Jackson and the Olympians,
The Heroes of Olympus,
The Kane Chronicles, some of
The 39 Clues series.)
He is collecting a series by Tui T. Sutherland (who has also written one of the Spirit Animals books) called
Wings of Fire. (Book 6 just came out this week; our copy should be here on Saturday.)
I almost forgot about Warriors by Erin Hunter; feral cats organized in colonies *must* be fantasy, right? The main novels are fairly short, but there are some longer "super editions" and electronic novellas too.
Wiki link to all series Goodreads link to the first series
I hope this helps. He is also a fan of the Middle School books by James Patterson, the Big Nate (not comic strip) novels, and the Diary of a Wimpy Kid books but those types of series do not seem to be as prevalent as fantasy ones.