Boy, I wonder if I could ever go to WDW often enough to get bored with the Magic Kingdom. Maybe if I lived in Orlando... nah, not even then probably!
By goodness, that poor kid is going all the way to WDW for the first time, and you're even thinking of not taking him to the MK??? Sure, a teenaged boy isn't going to care much for Main Street, Toontown fair, or Fantasyland, but Disney's mountain range, Pirates, Haunted Mansion... and just the overall ambience and feel of the place... it's just something you can't get anywhere else on Earth. Take him.