Point to the part in the article that says the mother has taught her son that long hair = girl and short hair = boy and he must aggressively insist that to be true to any passing tomboy.
You can't, because you've made it up. Nowhere does it say that the mother has taught her child that children must conform to gender stereotypes, or that he's been taught to be rude to children who don't. Humans are really good at correctly sexing each other.
A mother of a disabled child wants to know that those taking care of him will not will not force him to go along with someone else's fantasies, and will not censure him for stating reality. This reality is especially important for disabled children who may need intimate care and who are at increased risk of being abused compared to non disabled children.
That is more important than luxury beliefs about identities, or it bloody well should be to an organisation working with disabled children: you know, the actually marginalised and vulnerable.