For what it's worth, OP, I think there's something in the idea of calmly holding the line and I think you've done well to kindly but firmly say that you're not entering into this whole pronoun eggshell-walking while keeping the lines open with him.
Children really need their parents to firmly but kindly hold the boundaries of safety - but also reason and sanity - for them. Just like the transition phase of toddlerhood, a teenager getting the sense that there is no firm ground for them to stand on because the adults themselves are uncertain, unstable or frankly just a bit wet is quite terrifying. There's obviously a big difference between giving them a bit more freedom autonomy as they grow up vs effectively shrugging shoulders and saying, "sorry, child, I don't really know what to do and I'm a bit scared of your/society's opinion and reaction anyway, so you'd better be in charge and decide for yourself."
I do see a lot of parents these days who really do seem so unsure of themselves and slightly afraid of their children's reactions, as though they have no self-confidence that they know how to cope with them. Or they seem to know that they don't want to be shouty and unengaged like their own parents but haven't found a happy sweet spot (confident, kind leadership!) between that and permissive parenting.
It sounds like you're walking a good line now you've had time to think about it and not getting drawn into arguments about this either. Good for you. Good luck.
Agree that the school need to be set straight as well. Unless they seriously think that you pose a safeguarding risk to your child - in which case social services should have been involved - there is absolutely no excuse for them keeping this from you.
They are also not medical professionals and it has been established that "social transitioning" is not a neutral act. Therefore I would also be asking them on what medical advice, specific to your child, they were acting when doing this (and how they had gone about obtaining it without your involvement). The answer will be that they had none, in which case their administering this "treatment" in school without your knowledge is completely outside the bounds of professionalism and acceptability.