His injuries are not believed to be life threatening.
His mother Agnes Carson told The Daily Post her son - a second-year Rotorua Intermediate student - had suffered burns to 20 per cent of his body but would recover from his injuries.
"He's very lucky.
"It could have been much worse ... the mower petrol spilled and then exploded," she said.
The family were in shock but grateful to neighbours who called emergency services and ran over to pull the boy away from the blazing room, the woman said.
One family member, who didn't want to be named, said they were lucky not to be dealing with a death.
"We're just lucky it's not a tangi ... it could have been much worse," he said.
The 15-year-old's caregiver said he was proud of the teenager and Steffan for their quick thinking in pulling Richard to safety.
"They are heroes," he said.
Another neighbour, who didn't wish to be identified, said she was woken by a young girl screaming.
"She was yelling for help and when I looked out there were flames higher than the fence," she said.
Fire Service investigator Stuart Booten said the building was gutted by the fire and police were investigating the incident.
Constable Russell Bell from Rotorua CIB said the fire was accidental.
"It appears there was an open petrol container. One of the youths lit a lighter, causing the fire," Constable Bell said.
