Hi Kim,
This is really a tough question. I recommend you reach out to the wonderful cast members who work with
guests who have disabilities. These cast members can help you determine if you can have an exception to the dining policy or at least help you figure out whom to talk to.
If your son has to order from the adult menu or can not receive a larger portion, here are tips to help make meals less stressful. Visit AllEarsNet.com and look at the menus. This will give you options as to what types of food is available at a restaurant. You also might be able to take an adult dish and have the chef tweak it.
Next, look for buffets as here your son has no restrictions on which foods he wants to eat. I always it the chicken nuggets at Chef Mickey's because I like them. Plus who could pass up Mickey waffles for breakfast.
The other option is if they give you one child's meal on the dining plan you could always pay for an additional meal.
Finally, you can bring food into the parks which can always fill in the gaps.
Wishing you a magical vacation!
Beth