When we sailed in1/01 I bought my DD a Cinderella dress onboard. She loved it so much that I thought it would be a shame not to let her wear it. She wore it formal night and was a big hit, we didn't see any other "Princesses". Being only 3 at the time, she was oblivious to the attention she was garnering.
On semi-formal night we noticed a few "Princesses" had arrived onboard. It was so much fun. On our 1/02 cruise we saw so many more "Princesses" and realized the idea was spreading. I say - they are only a kid once, let them explore their imagination all they want. Before you know it they will be all grown up (I know, I have 3 adult sons and a 5 year old girl).
If your son wants to be Buzz, let him. After all, there isn't a better place than the Disney Magic to go "to infinity and beyond!"