Can you post a photo of the goldfish?
Goldfish have been bred into all sorts of weird shapes. Pet shop goldfish often have a very mixed ancestry, meaning that they can come out looking quite different from one another, depending on their genetic makeup. So it could be just his genes. Or, on the other hand, it could be the site of an injury or infection or some problem.
If he seems otherwise happy and healthy, I wouldn't worry about it.