"He has four brothers so I thought it might be to do with that but then he said he wanted his hair cut short."
Because girls physically can't have short hair.
Men and boys cannot and should not ever have long hair.
Short hair is an absolute sign that a person is male.
Moreover, this has held true across the world for all of history.
Hair length is a way better indicator to a person's sex than anything else.
I worry a bit about all this as I have a GNC DD who wore "boys" clothes exclusively for a few years (and it was about being labelled boys rather than the style, this was when she was 8 or so), still always wears a more neutral / masculine style (AKA comfortable), has short hair and wants it shorter etc.
At 10 though she has already started her periods and her body changing doesn't seem to be freaking her out.
She does follow a lot of LGBT+ stuff on animo and I asked her to stop, lots of transboys on there talking about binding etc
I also asked her if she thought people could literally change sex (get a penis / vulva) and she looked a bit confused and went with yes...
I do think that this is the alternative culture of our day, and that's all AOK, blue hair, makeup on boys, call me Dave, whatever. The bit where I baulk is when the medicalisation starts.
I also have a lot of concern about why no-one seems to be making teh (to me) obvious links around female puberty in a sexist society / being an older girl in a sexist society, that girls have always taken steps to try to opt out, and the fact that girls are the largest quickest growing group of trans people (transboys).