Wearing a dress is not a life altering decision. Further, theres no implication here that the parent is enforcing that the child has decided to be trans. Its a dress and armchair psychologists on reddit are immediately attributing things to malice. Consider that maybe the boy simply liked the dress, chose it himself from the thrift store as the mother asserted, and chose to wear it himself. If the above is true, that doesnt mean the kid is making the active decision to be trans. Maybe he just wanted to wear a fucking dress. Im playing devils advocate because this thread turned into an immediate brigade of transphobia despite the fact that theres zero implication from OP that their son is trans, either determined by the parent or child, and people seriously need to cool their shit.

As for the bullying; I wholly agree with you, which is why I advocated for explaining to the child that its going to happen, why its happening, and how to approach it.