I'm not sure it would be worthwhile to do....
With all the DDP food (apps, entree, dessert for the adults), there's probably plenty for him to nibble at. And at buffets (or all you can eat offerings at places like WCC or Ohana's), he gets his own plate and he eats free. The worst you would probably find is paying OOP for a CS kids meal and/or TS kids meal...and over the entire trip I'm not sure, unless you were doing "menu" TS places every night, that the OOP cost would be more than the DDP cost. I'd sit down and work out the numbers (if you haven't, already) before doing anything....