At most Table Service restaurants where you order from a menu your son will be able to order from the children's menu. There have been occasional posts that this was not allowed by a restaurant or server, but it has been rare. Nevertheless, it would be good to ask at each restaurant before ordering.
If your son wants something from the kids' menu you can pay OOP for his meal and save his adult credit for additional meals or 2 credit meals/dinner shows. If you are planning to use his adult credit for the child's meal you could ask if the restaurant can upsize the portion for him.
He will be charged an adult credit at buffets, all you care to eat family style venues, fixed price meals (CRT, Akershus, the family platter at Whispering Canyon Cafe), dinner shows (Hoop De Doo, Spirit of Aloha Luau, Mickey's Backyard Barbecue) and the Fantasmic! and Candlelight Processional Dining Packages.