Some autistic people can learn life skills, but struggle or fail to perform them regularly or reliably - particularly when there is a disruption to their routine, an unexpected change, or a period of stress. Routines like hiding in their room are often an attempt to shut out unpredictability and to exert control in a terrifying world.
For many, the period between 18-30 can be the most dangerous - autistic regression (and even catatonia) can develop, caused by prolonged stress, autistic burnout, depression and long term mental health issues. Regression means they frequently lose the day to day living skills they acquired even as children/adolescents, and for some they never recover them.
The most important thing here is to treat the son's withdrawal and (probable) burnout and have strategies in place to identify when it gets worse, which may well happen if he moves out suddenly without a transition period. The last thing OPs son needs is to develop catatonia/autistic inertia.
He sounds as though he needs tailored intervention from a small team of support workers, and a psychologist who specialises in autism. Dad really should be stepping up to the plate and using his money to hire the best lawyers to access the support his son needs. The hell has he been doing for the last 27 years???
Autism is very complex. All autistics have a spikey skills profile, some more extreme than others (think of it as a pie chart with uneven skills distribution.)
I have a cousin in their 30s who has a BA in a very specific area (think of something random like Norweigian literature) and an Mphil, but needs daily prompting to wash/dress/answer a phone/read emails/write/manage finances.
They are currently unable to work due to inertia, selective mutism and frequent daydreaming/staring spells - even a shelf stacking job fired them because they simply couldn't coordinate themselves, work at pace or or stay on task. They can write for days without sleeping on their special interest, but emotionally and socially they're really like a precocious 14 year old in the body of an adult with pragmatic speech/language/ motor skills difficulties.
Most people have no understanding of autism without learning disability at all, as this thread shows.
As a developmental disability, it can have surprisingly more in common with dementia in the way it affects the brain/personality, executive functioning and reasoning skills. Their monotropism can become so severe it impedes on daily living tasks. Many get 'stuck' developmentally in early adolescence, despite having a normal/high IQ, without help to move through transitions from school/university to adult life.
Apathy and fear/anxiety is a huge problem, and it sounds like this is what's affecting OP's son. He may not be able to show it or communicate it, but he needs treatment for it, and most importantly, compassion. Autistic suicide rates are 9-11 times higher than the general population for good reason. It's hard to cope with failing to launch in a world/social system you struggle to comprehend or care for.