Tests suggested Zane Gbangbola from Chertsey, Surrey, died from carbon monoxide poisoning, but his parents dispute this.
They believe he died after floodwater became contaminated by cyanide gas that leaked from a former landfill site.
Zane's father,  Kye Gbangbola, was left paralysed by the same incident.
Senior Surrey coroner Richard Travers said although a number of experts recently-consulted had come down on the side of carbon monoxide as the "most probable" cause of death, they were not able to be more certain than that.
He said: "There is evidence that hydrogen cyanide was detected at the scene."
'No criminal case'
Speaking at a pre-inquest hearing in Woking, Mr Travers said the amount of carbon monoxide in Zane's blood was "significantly lower" than fatal levels, although he said this could have an explanation.
Late last year, Surrey Police said no criminal charges would be brought over the boy's death.
A petrol-driven pump was seized from the family's home as part of the investigation, but detectives said there was "no criminal case to answer".
Zane's father and mother, Nicole Lawler, of Thameside, Chertsey, have spent the last 17 months campaigning for further investigations into the death.
After the hearing on Friday, Zane's parents said: "This was an important step towards revealing the truth about what happened to Zane.  We now hope for a full and proper investigation.
"We should never lose sight that this is about a little seven-year-old boy, Zane Gbangbola, who died needlessly."
There will be further pre-inquest hearings on 5 October and 7 December, before the full inquest on 18 January.
The inquest into the death of a seven-year-old boy during heavy floods will consider whether toxic floodwater played a part, a coroner has ruled.