If the restaurant is a fixed price restaurant then he'll get to eat for free. At buffets you just get him a plate and feed him. At restaurants that are fixed price like CRT they'll bring him a plate of food (at CRT I hear it's buttered noodles).
At restaurants where you order off a menu and pay for the exact items you order, you have the option to just feed him off your plate. If you want to order him a side dish you should be able to do that though you will be charged for that side. Most places also have a dish of fruit as a dessert option (even if it's not on the menu) so you can ask for that though again you will be charged for it OOP unless you choose to use one of your dessert options to cover the fruit. Typically, the meals are pretty good size and if he doesn't even eat enough to make a kids meal worthwhile then he should be fine eating off of your plates.