A father who waged a 20-year campaign for the truth behind the death of his son in the swimming pool of the entertainer Michael Barrymore has died.

Terry Lubbock, 76, who was diagnosed earlier this year with cancer, made what he described as his “final appeal” in April for witnesses as he fought for a fresh inquest into the death of his son, Stuart.

The 31-year-old was found dead at Barrymore’s then home in Roydon, Essex on 31 March 2001, after a party.

Barrymore said that he was saddened to hear of Terry Lubbock’s death, adding: “Essex police must carry on to seek justice for the Lubbock family, which I will continue to fully support.”

Paying tribute to Terry Lubbock, his friend Harry Cichy said: “He’s died sad, because he’s died knowing people never knew the truth about what happened. But no one could have fought harder for their son.

“A new inquest was what really mattered to him. He had lost faith in the police. Sadly, he’s died not knowing whether there will be another inquest.”

The retired toolmaker and driver, who lived in Harlow, Essex, was divorced and had another son. He had urged others with information about Stuart’s death to come forward, and had said in April: “For me the clock is ticking, and time is running out.”

During his campaign he produced a DVD and co-wrote a book, called Not Awight.

In 2009, a report by the Independent Police Complaints Commission (IPCC) upheld six of Lubbock’s 36 complaints about the Essex police investigation.

It found that two implements photographed by police at the scene – a 12in pool thermometer and a door handle – were never subsequently found or accounted for. Two senior officers offered an apology on behalf of the force.

“The whole thing has been covered up. We’ve got to dig a lot deeper,” Lubbock told the Guardian in 2009.

No one has has been charged in relation to Stuart’s death. Essex police arrested a man in March on suspicion of the indecent assault and murder of Stuart Lubbock, and released him without charge in August.

A postmortem showed Stuart Lubbock had suffered severe internal injuries, which suggested he had been sexually assaulted. Alcohol, ecstasy and cocaine were found in his bloodstream. A coroner recorded an open verdict.

Barrymore, now 69, was arrested in 2007 but never charged with any offence. He subsequently sued Essex police, claiming his wrongful arrest had cost him about £2.5m in lost earnings, but court of appeal judges concluded he should receive nominal damages.