@JazzyContemporaneousNotes and @mumsandaunties:
The way I have approached it with my DS was by imagining his worldview as an enormous edifice. I am not going to dismantle that in a single conversation - much as I’m sure many of us were not brought to terfworld in a single leap.
I also try to keep in mind that if someone (anyone, but let’s face it, particularly a parent, and even more particularly my mum) were telling me that everything I knew to be true was false, I would push back SO hard, almost in principle. (Also remember that terfs are “the bad guys” and so in the mind of the righteous, you must push back and educate.)
So keeping those things in mind, I drip fed, and I tried to do it in the guise of asking questions and then very deliberately not bothering to insert my opinion, but allowing him to draw his own conclusions. It helps if the conversations are had while you are both doing something, and preferably if you don’t have to make eye contact (car journeys are v good).
So one conversation went like this: “You know those transwomen on the internet who like, roll around and moan and say how terrible their period pains are?” (I think DS did not in fact know about such people, but they served their purpose…) “Do you think they are doing it for like, a joke, which is totally offensive to women? (DS professes to be a feminist.) “Or do you think they actually don’t understand their own anatomy? Because you know, obviously to have a period you have to have a uterus, and they haven’t managed to successfully transplant one of those yet so…”
And then I moved the conversation on to something else. And didn’t say what I thought or ask what he thought, or indeed say anything further at that point. Plant it and then leave it. You can come back at some later point and do the same again, with some different “weird thing” you’ve “just noticed”. And just keep nudging at those bricks.
Also: Someone somewhere said about cult deprogramming, that one of the key tactics is to enable the cult member to see the cult/cult leader as ridiculous. When someone is in it you cannot logic them out of it. But if you can get them to glimpse the ridiculousness, it helps to crack the edifice. So try for things that would make a true believer look like a lunatic, if examined in isolation - like transwomen suddenly growing uteruses.
I’m really sorry you’re going through this though. It is heartbreaking to think that your child, who knows you and knows you are a clever, thoughtful, person, has just decided that because you don’t buy everything they’re selling you must be a bigot. Sending hugs and strength.