The best advice I can give, as someone who has been there with a trans child (albeit under different circumstances and in a different time period) is not to give it much attention.
At the moment there are a lot of kids coming out as trans, they see it online, their friends encourage them, and there are quite a lot who love it when their parents push back because then they victimise themselves and get lots of attention, and that's when they can start doing things without your knowledge, especially at 15.
Let your child have a haircut, buy clothes your child likes, and it's really easy to not use pronouns or gendered language at all with your child, I wouldnt be using he/him, but i would definitely be neutral. Discuss the problems with binders, and suggest getting baggy tops and maybe sports bras or something comfortable but minimising.
When your child has nothing to push back against then hopefully it will just be a phase that's grown out of.
Meantime while you're being supportive of the way your child looks I would also recommend finding programmes and books and films with strong, female leads, engineering conversations about brilliant women, great women who look androgynous, there are also lots of videos online of women who felt like your child and began transition, then detransitioned, I think those videos should be an absolute must see for anyone even thinking about starting blockers or T as, quite often, blockers are touted as 'thinking time' and it's absolutely not true.
There's lots of support online for a 'watch and see' approach, as linked above.
I know others will disagree with aspects of my approach, but it's very easy to say "put your foot down and say no", it's not so easy actually living through it.
All the best op 💐