How should parents handle an adult child's shockingly messy room after he moves out?

Messiness in the eye of the beholder so here is my definition:
The door does not open more than a foot because of junk on the floor. There are boxes of stuff piled up so passageways are barely wide enough to pass through. The floor is covered in layers of old laundry, old magazines (1994 to present), food wrappers, used tissues, returnable bottles and cans, dirty clothes, expired foods, old textbooks and homework, leaked AA batteries, unopened mail from 2005 to present, broken lap tops, crushed headphones, etc. to a thickness of 8 inches to 1 foot deep. It looks similar to rooms seen on the show “Hoarders”. Does that qualify as messy?

On the surface, he is competent and capable and has been living with his girlfriend for more than a year. When he visits home, his parents have asked him to clean up and he spends hours doing it but there is no visible difference. The place is still a disaster.
It is like he is overwhelmed and simply does not know what to do.

Should the parents start cleaning out the room? If “yes” should it be done with or without his permission and or knowledge?

Is there a possibility that there is a medical reason for this behavior? What can his parents do to help him? Do you know anyone in this situation?