Like someone said, if your DS has an absolute favorite character, that may be the most important factor in your decision, but if he likes them all and you are just looking for general advice -
DS loved Garden Grill in Epcot when he was 5. The character attention is very good, and the food comes to you, so you are not worried about missing the characters by going up to a buffet at the wrong time. (The restaurant does spin slowly through some of the land sets, though, so don't do a motion-sick ride right before you eat there!)
If you are morning-people, I would recommend Crystal Palace for breakfast, because if you have an early reservation, you can get into MK before it actually opens. That one is a buffet, but it's worth having to employ a little strategy in fixing your plates for the amazing feeling of the almost empty park and the fabulous pictures you can get then.