I recently asked DS (now 10, but 9 on our last trip) for his favourite breakfast, lunch, and dinner at WDW. If it's at all possible, I'll try to incorporate his choices into the plans for next trip. His choices were:
Breakfast - Breakfastasaurus character breakfast at AK
Lunch - Sci Fi at MGM
Dinner - 'Ohanas at the Polynesian Resort.
Just for the record, I asked DD (now 7, but 6 on our last trip) and her responses were:
Breakfast - Princess breakfast at Norway in Epcot
Lunch - Prime Time at MGM (she's never actually been, but wants to try it)
Dinner - 'Ohanas at the Polynesian Resort.
'Ohanas is great for adults, too, and children are welcome to eat the same food as the adults or order from a standard kid menu. There are hula hoop contests and coconut rolling contests for the kids, which both of mine enjoyed. I think if you request it in advance they'll arrange a cupcake or something for your DS's birthday, too. After the meal you can wander down to the beach at the Polynesian and watch the electrical water pageant, if it's scheduled.