The incident occurred in the village of Attalens last Thursday at a farm owned by a 44-year-old man around 5.15pm, cantonal police said.
The fire got started after the boy was playing with a cigarette lighter and accidentally set straw alight, police said.
A total of 35 local fire fighters and 15 soldiers from a nearby base responded to the blaze.
Given the culprit’s young age, police said the file on the case will not be sent to the judicial authority for minors.
The injured man, a friend of the boy’s family, remains in hospital where he is being treated for severe burns.
He received the burns while attempting to put out the fire with an extinguisher, police said.
While the injuries are serious, his life is not in danger.