It's not in Magic Kingdom (I don't love the table service options in MK), but Chef Mickey in the Contemporary (right next door- very easy access) is fun and great for a 5 year old boy. The food is a buffet, so they get what the like/want and everyone gets full and happy by interacting with the characters. Great way to end a trip!
Or, there is Crystal Palace IN the MK, with Pooh and Friends, if he's into those characters.