You are looking at this entirely from one side. It is unthinkable for me, and has been ever since I was present at his birth, to deny that he is my son. My son, who I have done my (flawed) best for throughout his childhood, teens, and adulthood. The idea that I should obediently repent of my wrong thoughts because he tells me, against all evidence, that he is really a woman ... is ridiculous. You appear very concerned about his feelings, but you also appear to have no concept of my or my wife's pain. The cognitive dissonance of calling him "she" is probably outside your experience.
You talk about "deadnaming". If that term is valid, then it means that we are experiencing a bereavement. But it is a bereavement with no closure, no funeral. It is a bereavement that we are expected to rejoice in, because isn't it wonderful that we now have a daughter! We are to rejoice and express our delight if he starts taking oestrogen. If he goes down the surgical route, how wonderful that "she" is expressing "her" "authentic" self. The harm to his body is to be ignored. But we know the truth, that he is still the man that he was before his thinking was muddled by the highly sexist genderist ideology that he has embraced, the ideology that tells him we hate him if we do not convincingly pretend that he is in some nebulous sense "a woman". The ideology that tells him he must cut himself off from his family, including his wider family. The ideology that makes him a victim.
He can be as "feminine" as anyone else in the world and we will still love him. I appreciate his gentleness. I also recognise that in reality he is, like the rest of us, a mixture of "masculine" and "feminine". I too have some aspects of my personality that some see as "feminine". It doesn't alter the fact that I am male. Just as women have to deal with the reality of being female, both the physical reality and the societal expectations and limitations imposed on them, we (my son and I) have to deal with the reality of being male. He has chosen to deny that reality and through wishful thinking attempt to live his life as a woman. I will not lie to him and tell him that this is all good, and that he has my blessing in this utterly futile exercise. He cannot experience life as a woman, because he is not a woman.
I also worry that he may be led into some of the more sordid reaches of queer theory. There are enough transwomen who clearly are obsessed with unhealthy sexual practices for this to be a valid concern. But I am to affirm his illogical and destructive worldview no matter where it leads? And what about the impact on women? If I pretend he is a woman by using the pronouns he now demands, I am affirming his "right" to be treated as a woman and to be afforded women's rights. I am implicitly saying "you are a woman" and "you have a right to enter women's spaces". I will not do this. Though he can act as he chooses, it is not my role to approve his every choice.
Finally, he is autistic. That means that in some respects he is a vulnerable adult. His current main "special interest" is gender identity. He is highly vulnerable to being led into a more extreme version of genderism. I'm probably autistic too, though undiagnosed. I certainly share many of the traits which led to his diagnosis. I have felt the pull of religious cults. That he is now identifying as part of a grouping which includes advocates of extreme body modification and "unusual" sexual practices is a huge concern. And there are signs that he is being coercively controlled. Do I affirm his illogical belief, or do I stand up for physical reality? Like the story of the Prodigal Son, I am waiting for him to realise that his new friends don't love him as his family do, and for him to escape the illusion and come home.