Is he ever an inpatient? Or dose the service he attends have support staff. Check out any letters you have from the consultant and see who else is on staff. If there is a social worker contact them for help in applying for disability allowance for your son.
At 17 he is autistic with sight issues, endless hospital appointments. What are the realistic prospects of him getting the leaving and being able to hold down a full time job? Probably very low due to his permanent issues, so he should be eligible. Maybe he is not blind enough or not autistic enough for social welfare, I can’t tell but apply and if they refuse you can find out how much weight they gave to each element of his disability. The money would be so much help for food, keeping your car going to take him to his appointments etc.