If it were the boy's choice the parents should support him and the school should do something about the bullying. But since it seems to be not the boy's choice but the parents to dress him in ambiguous clothes to "fight gender stereotypes", they have culpability in the boy not feeling well in school. Being coerced to wear clothes he doesn't like to then be bullied for that as his parents try to make him a martyr of their struggle against gender stereotypes.

If is not the boy's fight, leave him alone.