It seems clear to me from what you've written, OP, that the underlying issue is your ds's fragile mental health - the transition thing is just a symptom of that.
So I'd address the source not the symptom. Hug your ds, support him as he needs your support as his mum now more than ever, to address the mental health concerns.
Mental health issues don't come out of nowhere - what led to this period of poor mental health? What can you do to help resolve the underlying issues? As well as helping him get counselling for that (NOT for his supposed 'gender identity')?
Make it clear you love him and are there for him. On the trans issue, can't imagine you need to use any pronouns other than you or your when talking to him directly, so that's that's a red herring.
I follow lots of detransitioners on Twitter and so many have parents who refused to affirm them and that challenge (eventually) helped them to come out the other side. In all cases, they were grateful and apologetic to the parent who had held their ground. Changing sex is impossible. Good parents don't deny reality.
I'd also be really worried about their taking hormones. That on its own is going to seriously mess with their mental health AND their physical health. I'd share lots of resources about the huge health implications of taking cross sex hormones - from the perspective of a caring, worried parent.
I'd try to understand what they think they're 'getting' out of being trans, and do my best to provide that 110%. So if they want to be cool and have a great social life, I'd support them in this. If they wanted permission to wear gender non-confirming clothes I'd support them in this. If they wanted a way out of their depressing life, I'd support them in this.
But I'd make it clear that no-one can change sex and the solutions to their problems lie in reality not fantasy, and that I wouldn't and couldn't go along with a misogynist fantasy. But I'd say that while hugging them and making clear this came from a place of caring and love and not rejection.