Your 11-year-old has read the Chronicles of Narnia six times and is weary of Potter-mania. What to recommend for a young fantasy buff? Here are a few laudable alternatives (available in various editions):
Lloyd Alexander, The Chronicles of Prydain
Madeleine L’Engle, The Time Quartet (A Wrinkle in Time; A Wind in the Door; A Swiftly Tilting Planet; Many Waters)
George MacDonald, At the Back of the North Wind; The Princess and the Goblin; The Princess and Curdie
J. R. R. Tolkien (trans.), Sir Gawain and the Green Knight; Pearl; Sir Orfeo