Kevin Hicks, 16, went missing in 1986 while walking home from the local shop in Addiscombe, Croydon.
He has never been seen since and in 2016 the Metropolitan Police said they believed he had been murdered.
Kevin was described as a happy teenager with no problems at school. He had been applying for jobs in the weeks before his disappearance.
On 2 March 1986, he spent a normal day with his family at their home in Sissinghurst Road, Addiscombe.
At around 8.30pm he told his mum he needed to buy some eggs for his O-level home economics exam the next day.
He left with £1 and headed off to Sperrings community shop, a couple of minutes walk away in Lower Addiscombe Road.
Kevin was last seen by someone who knew him at 10pm in Shirley Road, walking in the general direction of home.
He was never seen again and his parents Derek and Terry died without knowing what had happened to their son.
By 2016 his only surviving relative was his sister Alexandra Hicks, living in Sutton.
Christmas is always a particularly hard time of the year, especially now both my parents are gone. It would be the best Christmas present to have Kevin back or to know what has happened to him.
With Kevin and I being only a year and a day apart with our birthdays, the month of February is a constant reminder of not knowing where he is or what has happened to him.
On 2 March 2017 it will be 31 years since Kevin went missing and I appeal to anyone with any information, no matter how small, to come forward and assist police with their investigation. Someone somewhere knows something. It is time to let go of that secret.Alexandra Hicks, speaking in 2016
That year the Metropolitan Police issued a new appeal to the public and offered a £20,000 reward for information leading to a conviction.
Detective Inspector John McQuade, from the Homicide and Major Crime Command, said: “Kevin simply disappeared that night and was never seen nor heard of again.
“We can only imagine what the intervening years must have been like for his family, years of worry and distress at not knowing what happened. Sadly, both his parents died without ever discovering the truth.
“Many enquiries have been carried out over the years but Kevin’s body has never been found but there is no evidence he is still alive. We believe Kevin must have met someone that night, been assaulted and his body disposed of.
“A substantial reward is on offer for anyone with information leading to the discovery of Kevin’s body or the prosecution of the person or people responsible for his murder.
“I hope this will encourage anyone with information – no matter how small you think that piece of information might be – to come forward as soon as possible.
“We believe Kevin must have met someone in that hour-and-a-half and that person attacked him. Perhaps the suspect didn’t mean to kill him but it is clear Kevin’s body must have then been disposed of.
“On 25 October 1996, following media appeals, someone anonymously called the Croydon Advertiser and said she knew where Kevin’s body was. We would appeal for that person to come forward and contact us again; you may have really important information for us.
“I truly believe people in the local Croydon community hold the answers to what happened to Kevin.”
Anyone with information is asked to contact the incident room on 020 8721 4005 or Crimestoppers anonymously on 0800 555 111.