Nicola Cassidy said her son’s only contact with the outside world is through a hatch at Priory Hospital Cheadle Royal in Greater Manchester.
Ms Cassidy, 49, from Walton, Liverpool, claims “people wouldn’t treat an animal” the way her son is cared for.
The hospital said the 24-year-old’s care is continuously reviewed.
Patient A, whose name is not being disclosed to protect his privacy, was diagnosed as autistic aged seven, then later with Tourette’s syndrome and a learning disability.
He moved into residential care aged 14 after his family struggled to cope and was then moved to various placements before he was admitted under the Mental Health Act in September 2017 to Mersey Lodge ward at Cheadle Royal Hospital in a purpose-built apartment on the site of a former file room.
His story was reported in an investigation by The Sunday Times.
Ms Cassidy said: “We fully appreciate that my son has complex needs but he’s being treated terribly. He’s locked away from the world and has no physical contact with anyone.
People wouldn’t treat an animal that way and I feel that his care is worse than being in prison.
“Patient A has challenges but is a loving and caring person who needs stimulation and support. He is getting nothing at present. I can’t even hold his hand or hug him because of the conditions he’s kept in.
“This isn’t about money. He has five carers assigned to him all the time. That level of staffing is costly and is probably a waste of money given that he has no contact with anyone.”
She has instructed lawyers at Irwin Mitchell to investigate the care provided at the assessment and treatment unit and is seeking a community placement for her son.
A Priory spokesman said: “The welfare of the people we look after is our number one priority. “
He added: “Unfortunately however, some individuals with highly complex behaviours, and detained under the Mental Health Act, can be difficult to place despite all parties working very hard together over a long period of time to find the right setting.
At all times we work closely with families, commissioners, and NHS England to ensure patients are receiving the safest, most appropriate care in our facilities.
“Staff provide round-the-clock support at Mersey Lodge and all interventions are carefully and continually reviewed, monitored and assessed to ensure they are in the best interests of patients, with the aim of achieving the least restrictive setting possible.”
