A French couple whose son was killed in a hit-and-run accident in the UK six months ago have spoken of their devastation that police have still not charged anyone with his death, as backlogs in the forensics system mean the suspected driver has still not been identified.

Patrice Ippolito was 25 when he was killed in Wapping, east London, last October, and three teenagers were arrested after the crash. They were subsequently released because of lack of evidence about who was driving at the time of the incident.

The couple criticised delays in DNA testing that might reveal the identity of the driver, caused by backlogs at the laboratories the police use following the series of terrorist attacks last year.

Celine Ippolito said it was unbelievable that she still did not know what had happened to her son, who worked as a sommelier at Gordon Ramsay’s Maze Grill restaurant in Chelsea.

Two 18-year-olds and a 19-year-old were travelling in the car that hit Patrice, propelling him 30 metres (90 feet). He died before he reached hospital.

Celine Ippolito said she understood two cars had been travelling together when her son was hit and the three teenagers were subsequently arrested for dangerous driving.

“I don’t understand how the people in the car can live with themselves,” she said. “I don’t understand how they can sleep at night. I don’t understand how they can bear knowing that we, his family, do not know what happened to our son, because they won’t speak up.”

In a recent letter to the family, a police officer explained that laboratories had been overwhelmed with items requiring analysis following four major terrorism incidents last year and the Grenfell Tower fire in June.

“The results are taking ages to come back,” said Celine Ippolito. “The wait is terrible. We know that our son will never come back, we know he is dead, but we want the person who did it to admit what they did.

“I want to ask the people in the car: ‘How can you live with yourselves?’”

Patrice’s father, Serge Ippolito, said French law made it a crime to abandon someone in danger. Under the law, the three teenagers could have been tried for abandoning Patrice.

As home secretary, Theresa May announced in 2010 that the Forensic Science Service, a public body that provided services to the police, would close. Serious concerns have since been raised about DNA tests.

Celine and Serge Ippolito live in the Drôme region of southern France, and have two other children.

They said Patrice had loved London after moving to the city in 2012. He worked first in Le Boudin Blanc in Mayfair before taking a job at Ramsey’s Maze Grill. He is believed to have been walking home when he was killed.

“He was joyous. He lived life to the full, every day,” said Celine Ippolito. “He was a lovely man with a great future. He brought happiness with him. He loved life, and he was full of life.

“We are so angry at the people in the car. To me, it’s not possible that people can behave like this. If they would speak, it would let us move on. It is unbearable, not knowing. Patrice helped everyone, and these people won’t help in this small way.”