Hi Christine,
You do have a few options in order to accommodate your son. It really depends if you are on the Disney Dining plan or not. So lets breakdown the two dining options.
If you are going to be on the Disney Dining plan, you can leave him as a child and he will need to order from the children's menu. If he still is hungry, he can share your meal.
Or you can make your son an "adult". You would purchase him an adult theme park ticket as well as the adult dining plan. You have to upgrade to an adult theme park ticket as the dining plan is attached to the ticket.
If you are not using the Disney Dining Plan and paying "cash" for each meal, he is free to order off any menu and you will not need to upgrade his ticket to adult.
The Garden Grill serves the meal family style with endless plates of steak, fish, potatoes and vegetables of which some are grown right at the Land. Everyone is welcome to enjoy the feast.
Wishing you a magical vacation!
Beth