If you are paying out of pocket, anyone of any age can order off the kid's menu at any CS or TS restaurant.
If you are using dining credits, you can still order for your son off the kids menu at any CS or TS restaurant, but you may have to use adult credits to "pay" for the food (in which case, it might make sense to pay out of pocket for his meal and save the adult credits for another use).
Whether you are paying out of pocket or using the dining plan, at buffet restaurants, you will pay for your son at the adult price (or have to use adult credits) regardless of how much (or little) he eats and even if it is only food from the kid's portion of the buffet.