I'm not a mother, but I agree with the minority of posters who said the issue is not hair, it's the fighting.
Yes, kids can be cruel, but you don't hit people for making rude comments. That's completely unacceptable. He needs to learn how to appropriately address negative comments now rather than later.
With that said, I went to school for a little while in NO and my mother's family still lives there, and I can tell you it is NOT out of the norm to see little boys wearing their hair like your niece's son. Not even close. So if mother and son both like the braided ponytail styles, then I do not think mama should cut because he's being teased. I can tell you as someone with a gang of boys in their family that if it's not one thing, it'll be another. Weight, hair, preference in girls, family life, etc.
You can be perfectly groomed according to society's standards and still catch hell, but particularly with children, because many lack a filter.