Hi, Jenna!
Your son is welcome to order from the kids menu with the
Disney Dining Plan, but he will still be charged with his adult credit. It is not the best use of the plan, but I understand you just want your son to eat what he enjoys! Make sure he checks out the adult menu, as he might find something that he will like.
You may want to consider booking buffets. This way, your son can sample from all the food choices and might be a better value for your money. There are some great options including the Crystal Palace in the Magic Kingdom,
Chef Mickey's at Disney's Contemporary Resort, and
Hollywood & Vine at Disney's Hollywood Studios.
After each meal, your remaining credits will be conveniently listed at the bottom of your receipt. Your credits will expire at midnight of the day you check-out so make sure not to leave any credits unused. Your resort refillable beverage mug is yours to keep so take it home as a fun souvenir!
Wishing you many delicious meals during your visit to Walt Disney World!
Cathy