Do you have kids?
6/7 year old are really little. They probably believe in father Christmas!
You, and some other posters are acting as though what the OPs boy has said is irrelevant and just like any other incidence of unkindness.
It isn't; he and the other children are being gaslit to ignore the evidence of their own eyes . He knows he is being lied to by trusted adults, and he is confused. Punishing him for his confusion is going to be terribly harmful.
I'm not suggesting he should deliberately go out of his way to upset the trans girl, but people who are upset by reality are going to be upset sometimes. Its not reasonable to expect the entire world to lie because you can't cope with reality.
That poor child (trans girl), their parents are causing them a world of confusion and upset and setting them up for a lifetime of unhappiness - because they are being sold a lie.
Incidentally, there is no 'human right' , as a PPsaid/the OPwas told by school, to be recognised as the opposite sex (I can't stand people quoting made up 'human rights' to try to justify their behaviour):
https://www.un.org/en/about-us/universal-declaration-of-human-rights
However, the following articles may be relevant here:
18 Everyone has the right to freedom of thought, conscience and religion...
19 Everyone has the right to freedom of opinion and expression...
26(3) Parents have a prior right to choose the kind of education that shall be given to their children.