Tyler Doohan, from East Rochester, New York, was staying at the home of relatives in the nearby town of Penfield when he noticed a fire in the single-wide mobile home, Penfield Fire Chief Chris Ebmeyer said.
As firefighters and sheriff's deputies responded to Sunday night's emergency call, Doohan was able to wake six other people in the small trailer, including two more children aged 4 and 6, the fire chief said.
Doohan then went back to help his grandfather, who was disabled and would have been unable to get out of the home on his own, Ebmeyer said.
The pair was found together on a bed in the back room. It appeared that the boy was trying to lift his grandfather from the bed when he was overcome by the smoke and fire, the fire chief said.
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"It makes me really proud, it really does, but I just want him back," Doohan's mother, Crystal Vrooman, told CNN affiliate WHAM channel.
Doohan and his grandfather were like best friends, Vrooman said.
Another relative was found dead in a front room, although his exact relation to Doohan was not clear.
Several pets -- including dogs, cats, a rabbit and hamsters -- also died in the fire, Ebmeyer said.
Richard Stutzman Jr, interim superintendent of the East Rochester School District where Doohan attended fourth grade, issued a statement, saying: "In bravely and selflessly giving his own life, he was able to save the lives of six others -- and he truly is a hero.