Hi Natasha,
I would choose
Chef Mickey's! This fantastic, fun restaurant has a party atmosphere and is perfect for birthdays. Your little guy will see Mickey, Minnie, Goofy, Pluto and Donald. The buffet is fantastic and there is an "all-you-can-eat" ice-cream sundae station. Chef Mickey's is a top choice for birthday meals.
Another fun option is
'Ohana. They have coconut rolling, limbo and hula dancing lessons. My kids love eating dinner there.
If your son is a fan of the Hundred Acre Woods gang, then book the
Crystal Palace. Here you'll see Pooh, Tigger, Eeyore and Piglet. This is another buffet with a great selection.
Chef Mickey's is definitely my top suggestion though. Be sure to pick up a complimentary birthday button for your son at any Guest Relations counter. The Cast Members will love wishing him a Happy Birthday!
Hope this helps and have a wonderful vacation,
Kaylene