Campaign to help autistic boy, Matthew Garnett, who thinks he's in prison

Matthew is 15 and believes he's in prison indefinitely. He has autism, ADHD and learning difficulties.

For six months now, he's been detained under the mental health act.

He's in a psychiatric intensive care unit (Picu), which is for people with serious mental health illnesses and is only meant for short-term emergency care.

A four-hour round trip from his home in south London, his parents say staff there are doing their best.

But they say it is not equipped to deal with people like Matthew and it's making him a lot worse.

A spokesperson for NHS England (London) told Newsbeat: "We have every sympathy with Matthew and his family, and we and his clinical team are making every effort to ensure that he receives the most suitable care as soon as possible."

Matthew aged 13

He doesn't understand why he's there or why all the doors are locked.

He thinks he is in prison, being punished.

His family are only able to get down to visit once or twice a week.

Matthew and his dad in happier times

Since arriving he's put on a lot of weight. His mum says he's had no exercise or access to education. He's had to have his head shaved because he keeps pulling his hair out.

Matthew can become aggressive when he's upset and recently in one incident he fractured his arm.

So far he has pulled the cast off four times. Like many people with autism, he has sensory sensitivity.

His father Robin Garnett explains: "Having a cast on your arm is uncomfortable for anyone but for him it's agony. Light touch is like searing pain."

His mother Isabelle adds: "It takes some doing to get a cast off, so you can imagine the kind of pain he is in."

Robin says that Matthew's violent behaviour is a direct result of his autism and and (as yet un-assessed) mental health difficulties.

"Any child with autism has to a greater or lesser extent a difficulty recognizing, understanding and managing emotions. Matthew's degenerating mental health means that he is now only capable of instinctive 'fight or flight' under stress. Obviously flight is not an option for him right now and this is adding to the agony for us. "

When Matthew was sectioned, Isabelle and Robin were told it would only take six weeks before he could be moved to a specialist place.

Isabelle and Matthew before

That was in September. Now Isabelle's feeling desperate.

"There's nothing left I have now other than fear and fury and grief and I want to use it," she says.

"I can't even see where my son sleeps. He could be sleeping on the floor for all I know. For a mother it's agony, not to be able to protect your child on the most basic level."

Matthew and his dog

She says her natural response is to take him out of there, but that would be breaking the law. She's started a petition on change.org to get him the help he needs. Tens of thousands of people have signed it. Even the staff who look after him have signed it, she says.

Meanwhile Robin says Matthew has deteriorated, and "our mental wellbeing is being severely tested. No one is doing anything.

"The problem is a lack of provision generally and a lot of NHS bodies aren't joined up. No one wants it to come out of their local budgets. There are thousands of young vulnerable people like Matthew."

The family before

They have always struggled to get care for Matthew and prior to him being sectioned, there were no places in London where he could be assessed for specialist treatment.

Although there was a bed at one hospital that technically catered for Matthew, they were told it wouldn't be a good idea to leave him there.

Isabelle recounts the incident.

"I was told by a consultant: 'He will be eaten for breakfast. My wards are full of young people with skunk-induced psychoses'

"My son has an IQ of 55 and he likes the Teletubbies. We couldn't agree to admit him there.

"It's a scandal that we don't have provision for these people - it means they are criminalised and that's what happened to our son."

They ended up calling the police to try and get the support they needed. Isabelle says they were brilliant but that she felt their job was to fight crime - not care for her son.

Yet it was one night that looked like "the kind of thing you see in films" that led to Matthew being sectioned.

While they were waiting for someone to come round to assess him at home, Matthew was very unhappy because he had been taken out of school that he liked and he does not deal well with change.

Robin explains what happened: "He started punching me. I was trying to restrain him, but he was headbutting me. I've still got the bite marks. All the time he was screaming: 'I want to cut your head off, I want to kill you'.

"Eventually the police arrived and put cuffs on him.

"But by the time the people arrived to assess him, I was sitting on the front steps in tears, blood dripping from various places and the police were standing over my son in handcuffs. The kind of thing you see in films but this was real life. He got sent to the nearest place that would accept someone that has that record of violence."

Robin and Matthew

Matthew doesn't understand why he's in the unit. He doesn't think he's unwell.

He tells Robin: "It's because I hit you isn't it dad?" so he thinks he's been punished.

"Matthew needs to get to another place that can help him before he can go back to school and back to home and normality but we can't tell him when that will be."

Robin says it's very expensive to care for Matthew at the Picu and that taxpayers' money is being wasted.

"In fact it's being spent to make it worse," he says.

Matthew and his mum

Isabelle is disgusted.

"We're taking legal advice now because I just think where are his human rights in all this? This isn't just Matthew. That's what so disgusting about all this. And it all goes on behind closed doors and there's no bad guy because no one is accountable for it. What we need is action. These vulnerable young people are being abused."

A Department of Health spokesperson said: "It's crucial children with mental illness get the right care in the right place - change is already underway to make that happen. We are investing £1.4 billion into young people's mental health and are working with local areas to improve services so young people get better quality, preventative mental health care as quickly as possible."

Cygnet Hospital in Woking, where Matthew is staying, say they can't comment on individual cases but gave Newsbeat the following statement.

"We support some very vulnerable people, to whom we have a duty of care, a key part of which is respecting patients' confidentiality. Therefore, it would not be appropriate for us to comment on an individual case.

"Our primary purpose is to assist individuals with crisis support, stabilising them ahead of admission to a clinical mental health treatment and support service. Where a placement at a specialist service is not immediately available, a clinical decision is taken which may decide the most appropriate alternative is for the individual to remain in our care until a space does become available."

For more stories like this one you can now download the BBC Newsbeat app straight to your device. For iPhone go here. For Android go here