Honestly I sit on my hands almost all the time at these threads but I just can't tonight.
Disclaimer: not a parent. Animal behaviourist.
A 2/3 whatever year old who has only ever toileted in their living memory, against the absorbent fabric of a nappy, is conditioned to toilet in a nappy.
They have not experienced ANYTHING else. In fact sometimes, the feel of that fabric against their bums/bits actually tells them 'it's safe to let go now'.
And then you take that off, sit them on a precarious, too high up seat with a fucking big hole in it into cold cold water that disappears off who only knows where...
Yeah, I'd tense up and not let go too.
Imagine for a second that I ask you to ignore all your conditioning to going on a nice comfy toilet seat in a private little room.
Lets say I've asked you to piss your pants in the middle of the pub, or drop your trousers and do a poo in the middle of Kings Cross station, at rush hour.
Seriously, how many could actually do it and do it quickly, and happily, without a moments anxiety?
I would bet almost none of you. Because you've had decades of conditioning to ONLY toilet in VERY specific contexts.
Just look at the number of threads posted about not being able to pee outdoors, or not being able to do a poo at work or in a public toilet etc etc. That conditioning is SO strong, it's incredible.
Now think again about the small child who is being asked to do something completely new, and absolutely does not have the communication skills to explain the above to you, about why it feels SO wrong, and SO alien and SO scary.
OP - you aren't soft, you are thoughtful and kind. Do not let others bully you into ignoring your instincts.
Break the job down into stages - get a potty, make it familiar and generally nice, if you really need to, start some games of things go in the potty, doesn't at this stage matter what the things are, just the notion that 'something goes in there and then gets tipped out'. Water will do. Water and bits of tissue even?
So now there is no mystery about that - make it fun, make it rewarding and take off the pressure.
Then build the 'sitting on the potty' idea, build it into a game, 'can you stand on this, sit on that, lie down on that, stand on one leg, sit on the floor, sit on the potty, sit on a chair..'
Whilst this is going on, talk to him about the whole going to the toilet thing, about knowing when you need to go, what that feels like, how do you know, etc. Reward him for telling you he is peeing or pooing in his nappy, reward even better if he can tell you BEFORE he has to go (but still don't pressure him to go anywhere else).
I'd also give him some general discussion on where the toilet stuff goes - show him any pipework, tell him (in as simple terms as necessary obviously, he doesn't need a degree in plumbing) where the water from the loo goes, where the water down the sink goes. Make it normal and not scary.
Get him thinking about his body and how that feels - up until now, he has not HAD to be aware of it, it just ... comes out, so thats new.
Then when he is telling you about it, when the potty is familiar, and safe, you can start to offer him the CHOICE... to go on the potty with a bare bum for a bit, or stay in his nappy and sit on it...
Keep going like that, the more you can give him choices (they're not really FREE choice, they're choices between 'this and that' not 'do as you like', its more an illusion of choice!) and stack the odds in favour of him going for option you want, the better.
All this force and shouting and making it a huge deal is just going to build a horribly negative association not just with the toilet, but with his parents teaching him ANYTHING at all. Regardless of whether he has SEN, the force and the aggro will only make matters worse.
Ok, burn me with fire, I'm out.