Hello Jill! Does the dinner have to be with characters? If not, and you're looking for fun, I think you should give
Whispering Canyon at the Wilderness Lodge a try. The atmosphere is fun and rowdy with lots of antics - and don't be surprised if the entire restaurant sings happy birthday to your son! Another non-character location that is a ton of fun is
50's Prime Time Cafe.
If you'd prefer your dinner to be with characters, you can check the
restaurant page and filter on "Dining Experience" and then "Character Dining." And if your heart is set on Chef Mickey's, you can keep trying to see if there are cancellations.
Whatever you decide, make sure you note the celebration on your reservation. Also, have your son wear his birthday celebration pin (you can obtain at your resort or Guest Relations in each park) so your son will receive tons of Happy Birthday wishes throughout the day.
I hope your son's birthday is truly magical! Diane :)