Ah yes, our guy would also eat fried chicken and french fries at every meal if we let him!
I would think this could be treated like any other dietary need; TS places seem pretty willing to work with you, especially with advance notice. Call a couple of days ahead to discuss your son's needs or at least ask to speak with someone when you get to the restaurant about accommodating his eating needs (small portion, plain food). You may find a Guest Assistance card helpful, although we've managed for several years without one.
My son is 21, with autism. He ate at Boma last year: chicken nuggets, macaroni and cheese, lemonade, fruit and two chocolate chip cookies. Of course we had to pay the adult price, but many other teens/adults were eating like he was, so he didn't stand out.
We normally eat at CS in part because of our son's dietary issues--just don't want to be spending $$$ when he really only wants some fried chicken/cheeseburger/pizza. He loves the kids' meals so we get those often, and believe me, no one will chase you to your table to see if a kid is eating it! At the food court at Music (where we stay each year), I asked about adults being able to get mac and cheese and they served me a double portion, without any fuss. You also have more choices on CS meal sides these days, with fruit in place of fries even for adults. You can also order sandwiches without fries and normally save money--although fries are what our guy thinks of as vegetables.
Anyway, if you let the restaurant know your needs, whether TS or food court at your resort, they should be able to serve your son appropriately.