The 32-year-old, whose relatives wish to remain anonymous, is believed to have had cardiac arrest in the van and fell into a coma before dying two days later in hospital.
Police attempted to revive the man when they realised he was unconscious.
The man’s father, who did not want to be named, said his son had been spending his evening with his girlfriend and friends on Saturday, July 26, 2014 in Haywards Heath, Sussex.
When he fell into a complex partial seizure, a neighbour mistook his shouting for an argument and called the police.
His aunt said the officers arrived while as their suspect walked out of the house into the garden, unaware of his surroundings – behaviour that was routine for her nephew and is well-known among epilepsy sufferers.
She added: “He wandered out into the garden as the police arrived and they assumed he was violent.
“His girlfriend told the police he was having a seizure, she shouted at them to stop, but they ignored her.
“They tasered him twice and pushed him to the ground and three police officers sat on his chest to restrain him, at which point he had a heart attack.
“But they didn’t realise and put him in the van.”
They stopped the van in a nearby road and took the man out to administer CPR before an ambulance took him to the Princess Royal Hospital shortly after midnight on Sunday, July 27, 2014.
Relatives believe he had already gone into cardiac arrest and said he was unconscious in a coma until he died in hospital on Tuesday, July 29, 2014.
Sussex Police denied tasers were used in the restraint but would not comment on other allegations, including that pepper spray was used.
The force referred the investigation to the Independents Police Complaints Commission (IPCC), which launched a witness appeal and has sent officials to examine the scene and take statements from the officers involved.
A spokesman for the IPCC would not confirm details of the incident given by the family or that the man was suffering an epileptic seizure.
He said: “The nature of his interaction with the police and his restraint forms a central part of that investigation.
“A post mortem has yet to be conducted so it is not possible to confirm a cause of death or establish a medical timeline.”
In a statement released on Tuesday,, July 29, 2014 the watchdog said the man died after being restrained and “becoming unwell” in the police van but gave no further information on the condition.
The dead man’s father said the family were unhappy with the restricted information released.
“He was already having the seizure and we feel they used excessive force when he was on the ground,” he added.
“We feel that apart from our personal interest, there’s a huge public interest in this because it affects people with epilepsy and people with autism and other issues.
“We’re just trying to keep it together.”
Jennifer Izekor, commissioner of the IPCC, said the body’s thoughts were with his family and friends.
She added: “The IPCC will carry out a robust and thorough investigation into the circumstances and nature of his contact with the police, the events leading up to his becoming unwell and his hospitalisation.”
