Hi Kim!
Walt Disney World does a great job accomodating a wide range of food allergies and intolerances. Review the
Special Diets page on the Walt Disney World site for information on the types of food allergies and dining locations that can accomodate Guests' allergy requirements.
I also suggest that you contact WDW Special Diets at Special.Diets@DisneyWorld.com at least 14 days prior to your arrival to inform them of your grandson's needs and have them advise you of dining locations that can accomodate those needs.
You will also want to make a notation of this special diet on all Advanced Dining Reservations (ADRs) so that the chef can ensure the meals your grandson receives meet these needs.
I did a bit of research on this to find the best answer to your needs to help your grandson adhere to the Feingold diet while at Walt Disney World, and I found this great post on the nourishinghope.com website. Search on the site for "Disney World with Food Allergies" to find the article.
A mother writes about her experience with her kids adhering to the Feingold diet while at Walt Disney World. She has a very positive experience.
A chef or manager will come out to your table prior to your ordering and will advise you of the options availble at that location that meet your dietary requirements. Walt Disney World does everything they can to accomodate all guests needs.
It is also great that you are staying at the Cabins at Disney's Fort Wilderness Resort because the cabins have fully equipped kitchen's, which are important for preparing food and snacks that meet your grandson's needs. You can always bring your own food into the parks, which will be helpful for snacks in between meals.
Please let me know if I can help you with anything else.
Magically,
Stacy