We struggled with this with my middle child. He’s not autistic but has a language delay. We went to an occupational therapist and a psychologist as he had a real phobia about it all. Literally nothing we tried worked. Eventually he became scared of the change table as he was getting too big for it and that fear outweighed his fear of the toilet. Then it was literally overnight as he already knew exactly what to do from watching older & younger sibling, us, and all of the million techniques we had tried. He was 5.5. Typical of everything in his world – always on his schedule, when he is ready. No amount of encouragement, pushing, etc makes any difference!

I would get him to sit on his toilet near you while you sit on yours, keep praising about being big using it.
The other thing is when I worked with a Autistic child I would get them on the loo and we played a game as this child was none verbal we did clapping and it did help but it is hard work and you have to keep doing it or they won’t always do it.
Good Luck

I’d recommend using your NDIS funding to see an occupational therapist to assess the individual needs of your little one, he may just not be ready.

Otherwise you can try contience Australia’s webpage that that some great visuals and guides

My kids are not autistic, but every one f them refused to have anything to do with toilet training till they were ready – and then did it almost overnight. They were all close to 5 years. So maybe he’s not quite ready?