Maybe OP's son is on a gluten free diet. Or maybe, like a lot of 3 year olds, only eats certain things, lol.
I don't recall the size of the kids burgers, but if OP is a light eater, then maybe she could split a regular adult meal burger with her son.
We've always done well at the parks to get 2 adult meals and one kids, shared between 2 kids and 2 adults, but now that DS is 15, and DD soon to be 9, I think it might be harder to do this on our next trip.
There's always churros, to fill up on, in between meals.
Dizzy4DL, I haven't seen the kids burgers, but know that anything else in the kids meals is pretty small... seems Disney is thinking more toddler meal rather than kids meal. Remember too, according to Disney, you are an 'adult' once you are 10 years old, (paying adult prices at the gate.) Maybe they want the 9 and 10 year olds to start ordering a full adult meal. Best value meals for kids are at the Coke Corner for kids (turkey dog, etc.) as well as the protein power pak for around $6, with the yogurt, cheese string, juice, and goldfish crackers.