Hi there, Stephanie!
You are welcome to bring snacks and beverages into the Parks at Walt Disney World -- and into the restaurants.
Additionally, the chefs at the restaurants are incredibly accommodating and will work with you and your son to ensure that his meal is not only prepared to his liking, but is just as tasty as all of the food it at the World. There's
more information on the official Web site.
Male sure to ask for the chef immediately upon being seated. S/he will come to your table to discuss the options available to your son.
Because of the rules of the Disney Dining Plan, your son is already on the Plan, and should be able to use his entitlements to his enjoyment.
One thought: give a call to (407) WDW-DINE and make sure that you let the Cast Member know that you'll have some special requests. That information, already in your file, will help speed up the processes at the restaurants.
Have a wonderful time!
See you later, celebrator,
-Deborah B.