The elderly parents of missing Lee Boxell pleaded for information “before it’s too late for us” on what would have been his 49th birthday.
It is feared Lee - then aged 15 - may have been killed in 1988 after witnessing child abuse at a teenage hang-out near his home in Sutton, south-west London.
Crimestoppers on Wednesday offered a £20,000 reward to catch those responsible.
Lee’s parents, Peter and Christine, have kept his bedroom exactly how it was on the day he went missing nearly 34 years ago.
Police said the Cheam High School pupil used to visit an outbuilding at St Dunstan’s Church dubbed “the Shed”, a gathering place for teenagers that was targeted by sexual predators.
In 2014 three men were arrested on suspicion of murder. No one has been charged.
Mr Boxell, 75, said: “We’re desperate for someone to come forward and tell us what happened to Lee which may ultimately help find his remains if he was murdered as we fear.
“Crimestoppers really does give someone a chance to do the right thing and help ease our years of pain. We can then see Lee finally laid to rest.
“Tell the Crimestoppers what you know anonymously so that my wife and I can say goodbye to our beloved son before it’s too late for us.
“It’s a race against time now we’re in our 70s – but you can help us, and his sister, niece and nephew finally know what happened and have some closure - so please, do what’s right.”
Alexa Loukas, London regional manager Crimestoppers, added: “For decades, Lee’s disappearance and the suspicious circumstances surrounding what really happened to him, have haunted his family, friends and the wider community.
“It shocked everyone at the time and continues to do so to this very day.
“It may be that people who were teenagers at the time know what happened to him and have kept this secret for years.
“Maybe they are too scared to speak up about the tragedy, but with our guarantee of anonymity – which we’ve always kept – they now have the opportunity to do the right thing. We’re appealing for information that can solve this crime and help Lee’s family finally bring him home to rest.”
Anyone with information on Lee’s disappearance can call Crimestoppers anonymously on 800 555 111.