Nahuel Matias Santucho was travelling with his family to Buenos Aires so he could spend his "last days of life" with relatives in Argentina earlier this month.
The family arrived at the London terminal from Warsaw but missed their connecting flight meaning the 15-year-old, who suffers from an incurable illness affecting his brain and spinal cord, had to be "nursed on the floor", BBC News reports.
His mother Andrea Lopez said the group had missed their connection after a 45-minute delay departing their plane from Warsaw because of a problem with a lift.
She described how the family were taken into a corridor to wait for a new flight in which Santucho had to sit in a "very narrow and uncomfortable wheelchair".
During their wait, she said her son "started to moan and show evident signals of tiredness and discomfort" so was moved to the floor to be treating by a paramedic.
Lopez added that after "several hours" a British Airways employee arranged for the family to be taken to a nearby hotel and rebooked on a flight the following day.
A Heathrow spokesperson said it was "clearly a distressing time... for which we are sorry.”
They said investigations had found "Napu’s own wheelchair was not available once he left the aircraft and he was uncomfortable in the replacement wheelchair" so was moved to the floor.
"We continue to work with all parties involved to find out all the detail of what happened and how his journey could have been made more comfortable," the spokesperson said.
A spokesperson for BA said they had "provided hotel accommodation" and "arranged for a doctor to provide suitable food" but would "continue to investigate with Heathrow Airport and their wheelchair provider".