I guess that depends on how much your son likes the characters. Last May was our first trip and we were celebrating my dd 3rd b-day. We went to Chef Mickey's the first night and she absolutely LOVED it!! The food is just "OK", good but not great, but the character interaction with the children was worth the trip. Every one of the characters come to your table one by one to interact with each of your children, hug them, & take pictures.The birthday children do get their own placemat and b-day cupcake with a candle. They also have a little b-day song for them and swing their napkin around above their heads while singing. The whole restaurant participates. It was really cool. The look on my daughters face when she saw them was priceless.
We also went to O'hana's on that trip, and I have to say - it was REALLY GOOD food!!!They cook the food on these huge fire pits inside the restaurant and bring the food to your table. They also do lots of fun things for my ds9 in the restaurant to do (coconut races, limbo contests, hawiaan music, etc. ).
My ds still talks about how much fun he had at O'hana's and my dd still talks about Mickey and friends at Chef Mickeys. However, she is 3. Your son will be 6, and depending on how much he "loves" Mickey and the gang would determine for me where to go. If you can afford it, do both. You can celebrate his birthday at as many restaurants as you want to.
Hope this helps. And don't worry, he will have fun wherever you bring him. This is DISNEYWORLD!! Everything is fun!!