Devastated parents are seeking answers as to why their 23-year-old son died after being restrained at a mental institution by up to seven policemen.

The Independent Police Complaints Commission (IPCC) is investigating the death of Kingston University student Seni Lewis, who became agitated while in the care of staff at the Bethlem Royal Hospital, Beckenham, on September 1.

The police were called to restrain the young man, from South Norwood, who “became passive” and then “seriously unwell”, according to the IPCC.

He was taken to Croydon University Hospital after slipping into a coma, from which he never recovered. He died after his life support machine was switched off on September 4.

His 61-year-old mother Ajibola, a retired lawyer, said: “We don’t know exactly what happened. We do know they called the police and he ended up in hospital.”

She and Mr Lewis’s father Conrad and sisters Kemi and Lara await the outcome of an IPCC investigation.

Seni’s family said he was healthy and had never suffered mental health problems, but started to act unpredictably after a night out with friends over the bank holiday weekend.

Mrs Lewis said: “He went out with some friends and he had something to drink and he was smoking something that was not right.”

Seni had become alarmed about his agitated state and asked his parents and best friend Omari Faria to take him to hospital on August 31.

After speaking with a doctor, his parents decided to let him stay overnight at Bethlem Royal Hospital as a voluntary patient so he could recover and rest.

Mrs Lewis said: “We left him at 8pm with the understanding that if Seni wanted to leave at any time, the hospital should call me and I would fetch him.

“The next thing we had a call just after midnight to say Seni had been taken to hospital after an incident, but that he was OK.

“We think he got up and wanted to go home, because that is what he thought he could do.”

The family do not know when the investigation will finish, but are desperate for answers as to why he died in such a tragic way.

Conrad, 59, a former computer specialist, said: “We left him in the hands of the authorities with the assurance he was going to be looked after.

“We want to know he has not died in vain, that if there were any errors, they should be identified and actions taken to correct them immediately.”

Seni Lewis, who was 6ft 2in, was affectionately known to family and friends as a gentle giant.

The churchgoing 23-year-old, who was working to go to America so he could do a PhD in computing, was a hero to his nephews and a caring brother and son.

His sister Kemi said: “It has been a blessing to have him for as long as we did.

“We are devastated. It is only our faith that is keeping us going each day.”

His mum said: “Along this road he was known as a gentle giant. He was a bit of an old man, he liked to tell his older sisters how they should act and he was very good with his nephews, aged seven and 10 months.”

The youth, who did an undergraduate and masters degree at Kingston University, was a keen boxer who played football at school.

He was an adventurer learning taekwondo, scuba diving and loved to jet ski on family holidays to Florida.

He could often be seen cycling around London.

His mother said: “As a child he kept a tarantula as a pet. I remember one day it escaped and the kids all ran out the house leaving me to look for it.

“We are a Christian family and Seni went to Sunday school and then youth group. He was very strong in his faith.

“He was very considerate and loving and adored his 91-year-old grandmother, who lives with us.

“Our neighbour’s children were just devastated when he died because he was always looking out for them. He was no angel, but he was a good guy.”

His father said: “He was very talented. I looked at two of his computer projects and they were seriously good. If he had had the chance he could have marketed his ideas and made a lot of money.”

For more details visit justiceforseni.com.