Battle of Books

Devin is obsessed with the Percy Jackson books.  A friend gave him the first three of the series for his eighth birthday.  He devoured them.
For those of you unaware, Percy Jackson is a twelve-year-old demigod (meaning he's the son of a mortal woman and the Greek god Poseidon). Percy goes to a camp for demigods where he learns to leverage some of his talents, like harnessing the power of water (did I mention he's Poseidon's son?) and fighting with weapons and he befriends a satyr and other demigods.  Then Percy and his friends go on a quest to prevent a war between the Greek gods Zeus, Poseidon and Hades.  That is the high-level summary of the first book.  Devin has read all five (of the 300+ page books) in the series.  Then he asked about the next series by the same author.  This one continues with the Greek gods, but the books are even longer and the plots and the cast of characters continues to grow. 
I love it.  This kid will willingly pick up one of these books at any time and disappear.  I am thrilled that he has found a series that has deepened his love of reading.  I just wish that there were more books in the series.  We are down to the last book, and I am wondering what will be next.  Therefore, I have been encouraging Devin to branch out to other book series.  I have suggested Harry Potter and 39 Clues (my niece's recommendation) but he has no interest.  No interest in Harry Potter?!?  This is the series that made reading cool for a whole new generation of kids.  I have read every one of the books, and they are phenomenal.  I just don't understand it.  We have had this conversation more than once, and Devin remains resolute.  He is not interested in reading Harry Potter.
In the car tonight Andy was asking Devin about the Percy Jackson books, and he finally got to the root of Devin's issue with Harry Potter.  Devin said, "Percy Jackson is cool.  He fights with swords and water and stuff.  Harry Potter just has a wand."  When you put it that way, it sounds much less cool. 
So I am open for recommendations....