From reading several of his books, including his insightful autobiography/guide-to-writing "On Writing," I get the distinct "mature" ENTp vibe from him. He seems to get his ideas from a complex stream of intuition, which he then enriches through a childlike feeling of wonder and glee, things that I associate with your typical ENTp.
photos
quotes:
http://www.goodreads.com/author/quot...9.Stephen_King
http://en.wikiquote.org/wiki/Stephen_King#Quotes