Zane Gbangbola died at his home during last February's floods.
A post-mortem proved inconclusive, but a pathologist said he had been killed by carbon monoxide poisoning although no evidence was found and this raises more questions than answers.
His parents, Kye & Nicole believe he died from hydrogen cyanide poisoning from a nearby landfill site.
Ms Truss has now demanded action after meeting with the parents in November.
She has written to Paul Leinster, Chief Executive of the Environment Agency, formally asking him to test the lake on the landfill site yards from the family home.
Writing to Kye and Nicole's local MP, Kwasi Kwateng, she said: "As a parent I am extremely saddened to hear in our recent meeting more details about this tragic case…
"It is clear that on the face of it a number of institutions have some important questions to answer and I was concerned to learn about the slow progress that has been made by a number of bodies in getting to the bottom of what happened."
She said she had ordered the Environment Agency to "take a number of actions" and also ensured the Attorney General was made aware of the case.
"Firstly, testing of the nearby lake for chemical toxins, including chemicals that react with water to release hydrogen cyanide gas. The Agency will be happy to share details of the methodology, equipment and results," she wrote.
"Second, to investigate the parents' criticism of poor communication from local Agency staff."
She added: "Third, unless prohibited by law, that the Environment Agency share as much information as possible on this case with the family."
Responding to Ms Truss's letter, Zane's father Kye said: "This is what we have been calling now for 11 months, for the land to be tested behind our home.
"It has been that long and still we have no date for an inquest, no cause of death, and no answers over how our son died.
"We have not even been allowed to step foot back in our own home until gas proof membranes, ventilation works and gas detection alarms, are completed to provide the protection that the Environment Agency have on their neighbouring property.
"The Environment Agency protected their property five years ago without telling any neighbours. We couldn't even pick up some of Zane's belongings as it is a health risk due to the landfill gases still leaking into the Victorian property."
At first, police believed Zane died from carbon monoxide poisoning but his parents say this is impossible as their house was fully electric.
Kye said: "The Crown Prosecution Service has thrown out the police Carbon Monoxide applications and there needs to be a focus on the actual evidence as we have always said from the outset."
He added: "I am glad that Ms Truss is doing something about it. The government needs to ensure British people something like this never happens again. We have lost a son - yet thousands of other people may unknowingly be at risk.
"There needs to be an urgent inquiry not just into why Zane died, but into historical landfill sites in the UK and the consequences of them being flooded.
"Anyone who knew our Zane will know exactly why we need to get to the truth. He was a incredibly bright, generous, happy and life-loving boy who had such a wonderful future ahead of him. We cannot let his death be in vain."
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