I'm sure that most of the time you will be charged for him. It never hurts to ask. My aunt had cancer and because of the radiation to her intestines she can only eat certain things like soups, purees, etc. and small amounts at a time. In 2008, when we were there Chef Mickey's allowed her to bring a cup of soup from the neighboring counter service and did not charge her for the buffet. So yes, it is possible they will make an exception but I would be prepared to pay full price. You might be better off to choose a TS meal where you can order only what he will eat. I do understand that it means skipping the character meals because they are all buffet or family style. Obviously breakfast would be the best choice for you as it's cheapest.