I could have written your post.
It's the fear of not having the skill set or the mental energy to do all the things that seem so unreachable to them. I think the difficulty is that to them things are black and white. 'I'm 18 therefore an adult but um scared to adult because I can't do it'.
My youngest regressed a lot at 19. They went back to needing to be babied for security but this passed within a month.
If your son is at home and you are able to support him that makes a big difference.
Some practical things.
If you haven't already apply for a need's assessment. You get one via GP referring to social services disability team. This flags needs, it signposts too, and will help the transition and training if they are able.
Ask also for a carers assessment for you. This asks what you can do and are prepared to do with a time line. Looks at how you are coping.
You can ask for a limited work capability assessment. This, even if they don't qualify for benefits will acknowledge their limited capacity for work and cover their NI contribution.
Consider applying for PIP. This is exactly what it's for. This can be difficult to access unless you understand the process but there is lots of help to understand descriptors for each section. Stick to the discriptors and write it to include the relevant discriptor for each section. If you are not successful then appeal, take it all the way to tribunal with a specialist panel. The vulnerable especially people with autism really need supporting and the system isn't set up for this.
If they are transitioning to adult mental health services he can fill out a form giving them permission to liaise with you directly. Otherwise it's a nightmare to navigate.
Same with the GP and dentist.
My youngest refuses to clean teeth and gets frightened at the state of them. Recently discovered they can tolerate the hygienist at the dentist gently cleaning them for them. Still can't cope with the dentist but it's a positive step forwards getting into the building.
Wet wipes are you friend. If they can manage wet wipes for essential personal hygiene it's another step towards being clean.
I always say to my autistic adult children that the process of gaining life skills is slow. It's not instant, there will be setbacks. That we can do this together one step at a time. I say when things don't go to plan that it's a learning opportunity. We are all constantly learning.
My younger autistic (adult) child coped being in a quiet restaurant with a friend on Sunday. I booked it, i asked for a booth, I settled them, I ordered for them, then I sat outside with a glass of wine. They really wanted to try to do this together and both (autistic) young adults managed to tolerate the new environment for half an hour, without speaking to anyone and enjoy a pizza. I was in phone contact and being messaged by them.
I'd say that was a major step forwards.
It's about acknowledging every small step and also acknowledging that there will be many times it's a step backwards.
You are not alone.