Hi, Tisha!
Congratulations to your son. That is quite an accomplishment, and I love his choice of rewards!
I have always understood the minimum age to be 16, and several documents I have found state age 16. On the
webpage, it has both 18 and 16 as the minimum age (a few quirks are still being smoothed out on the new Disney website, sorry about that!)
I am fairly confident that your son will be just fine, but confirm when booking your reservation. Call (407)WDW-TOUR to schedule this very interesting walking tour of the Magic Kingdom. I did this tour several years ago and it was absolutely amazing to me to see all that is done behind the scenes to create that perfect Disney magic!
Enjoy your tour, and make sure your son gets a free "I'm Celebrating" button to mark his wonderful accomplishment.