There have been no confirmed sightings of Lee since he was seen on Sutton High Street on September 10, 1988 - leaving his parents in agony and without answers for more than three decades.
His father Peter Boxell, who is now aged in his late 70s, said: “We fear that Lee may have been murdered. We are still hoping that someone will come forward to help find his remains, so that my wife and I can say goodbye to our beloved son before it’s too late for us.
“If you can help us and his sister finally know what happened and have some closure - please, do what’s right.
“If you know where he may be, or have any information about what happened to Lee, please contact the police Investigation team on 0208721 4005 or if you prefer, you can tell the charity Crimestoppers what you know, anonymously.
“Crimestoppers can’t identify who you are and will never ask or take any personal details from you. So, this really does give someone a chance to do the right thing and help ease our years of pain.”
Lee left his home in Sutton to go shopping on September 10 in 1988. He met with a friend and they parted ways around 1pm.
The last confirmed sighting of Lee was at Sutton High Street at around 2.20pm.
His movements on the day he disappeared have been reviewed during a long-running police investigation.
Detective Chief Inspector Kate Blackburn said that while the Metropolitan Police does not have “conclusive evidence that Lee came to harm”, the force believes he is no longer alive.
“Over the years we have interviewed a number of people who we believe were involved in, or know about Lee’s disappearance. When arrested, those people gave a number of conflicting accounts,” she said.
One man who is now dead claimed that he had “helped Lee to get away” and that he was now living away from his family under an assumed name.
“This version of events was investigated and was found not to have been credible,” Ms Blackburn said.
“This man, and his associates, also claimed that they had seen Lee a year later. However, members of the public called police anonymously with information stating that he had bragged that he had buried Lee in the churchyard at St Dunstan’s church in Cheam.
“That information led us to conduct the largest ever archaeological dig undertaken by the Met at St Dunstan’s church yard. Sadly, that search did not find Lee.
“Lee was a boy with a very happy family life. He was a good student who loved football. The story that he had run away, started a new life and was living under an assumed name was absolutely not viable.”
Information received during the investigation also led officers to believe that Lee visited an unofficial youth club at St Dunstan’s Church known locally as “the shed”.
Local children and teenagers would have visited “the shed” and “those children are now adults who may have vital information that they have not been able to share in the past”, the Met said.
Ms Blackburn added: “My hope is that someone who didn’t feel they could speak to us in the past may now feel that they can come forward and share what they know.
“At this stage of our investigation my priority is to find Lee so that he can be returned to his parents. Lee’s parents deserve answers and they deserve the opportunity to bury their much-loved son.”
A reward of £20,000 has been offered by Crimestoppers. To be eligible for the reward the information would need to be provided to the independent charity Crimestoppers direct and not via police.