Harry Potter Claims Another Fan

Like a wise mother, I've been keeping Harry Potter for the right time for my now 11 yrs. old son.
The criteria I used had less to do with reading level as with eagerness.  I wanted him to want to read it, I mean, really, really want to.
I wanted his curiosity to be so peeked that letting him read the books would be like a rite of passage for him. 
I started six months ago by attacking from where he would feel it most: Legos.  We went to Lego.com and I showed him one of the biggest stets, the Lego Harry Potter Castle.  We told him that he could earn this castle if he did a whole school year worth of math in half a year.  Once he got it, he wanted to know what all those rooms and secret passages were.
He watched me take my nieces and nephews to the last movie, and he was so jealous that he didn't get to go--the big rule in my house being--read first then watch. 

Last week, I unleashed my son and let him check out the first book from the library.  It was a thing of beauty to watch a child with ADHD sitting...and reading...all day long!  He brought the book to the dinner table, he read in the car.  He stayed up late, skipped on whatever we happened to be watching on T.V!  Now he's done with the first book and begging me to take him back to the library for the second.
I have to admit, that part of me is a little sad, because I know that he will breeze through these books and then they will be over.... 
He will experience that lonely, disoriented feeling of, "what now?" 
He's read the Percy Jackson series, and really enjoyed those.  But I need more!  I need to keep that momentum going and I need new series to fill that void, and quickly! 
Please Help!