you can pay to upgrade your son to a Disney "adult"...this means you'll have to register him as age 10, and pay for an adult ticket instead of a child's ticket, but that's usually not all that expensive a difference. Then he can order from the adult menu. It's not a huge difference in cost -- Also, there's a few places that have no kids' menu so he can order the adult meal there -- however, those places are limited-menu places like Casey's Hot Dogs, Turkey leg place, fish & chips in Epcot, and so on. It's more of an adult-sized serving, but not much choice at those few places.
Or, you can just keep the
free dining, and buy the occasional extra item when he's more hungry. That might be cheapest way to go.
They will not let you pay the difference between the kids and adults meal plan (which would be quite a bit, anyway...much more than paying for the difference between adult/child park tickets.)