Zane Gbangbola, who was seven, would have become a teenager on 21 October.
A coroner said he died from carbon monoxide poisoning but Zane's parents insisted he was killed by landfill gases and want their evidence examined.
The Environment Agency said there had been a public inquiry where the coroner had given a comprehensive decision.
Parents Kye Gbangbola and Nicole Lawler have won the support of trade unions and opposition MPs, including shadow justice secretary Richard Burgon, who has said a Labour government would hold an independent panel inquiry "to finally get the truth about Zane".
At the recent Labour Party conference, Mr Burgon told a meeting of lawyers: "People shouldn't have to fight for justice, but in the society we live in, sadly, all too often they do."
As the date of Zane's 13th birthday approaches, his parents remember him as a child who at a young age campaigned for the environment and cared about the planet.
He was also a boy who loved sports cars and whose favourite place to visit was Brooklands motorsport and aviation museum.
Calls for an inquiry into his death have come from the Fire Brigades Union and an Early Day Motion by Labour's Jess Phillips has been backed by 50 MPs.
A spokeswoman for Surrey coroner Richard Travers, who in 2016 concluded Zane's death was accidental, said it was not appropriate for him to comment.
The Thames burst its banks in February 2014 but Zane's parents said the road between the river and their house remained dry, while their property was flooded by water from a former landfill site behind their home.
The evidence they have gathered is listed as covering medical reports, statements and test results, some of which has been shown to the BBC.
It also included archives documenting munitions-testing on the land in the 1940s and waste-tipping in the 1960s.
- FBU backing for flood death inquiry
- Accident caused boy's flood death
- Flood death boy's parents demand inquiry
They said they believed the authorities knew the hazards and their son's death was "preventable".
They said there would be "massive" implications for the government, insurers and public if it was accepted Zane died from exposure to hydrogen cyanide gas from the former landfill site.
Mr Gbangbola said they wanted an independent panel inquiry because it could compel full disclosure by the authorities and he added: "There was absolute knowledge of the risk to life."
Ms Lawler said the 2014 floods that saw their son's death became the first incident linked to climate change by a UK prime minister, and she warned: "Floods are increasing and the climate is changing. This will happen again."
The BBC has approached the landowners for comment.
In a statement, the Environment Agency said: "There has been a public inquiry into the cause of death of Zane where the coroner provided a comprehensive decision and report on the inquiry. We have no further comment."
A spokesman for the Department for Environment, Food and Rural Affairs said: "This is a tragic case and our thoughts remain with the Gbangbola family.
"Throughout the inquest the Environment Agency provided detailed evidence to assist the independent coroner in reaching his conclusions."
Original article >>