Police believe Ty Dodson was killed by his grandfather, Terry Majors, 64, who also murdered his wife Leigh Shea-Majors before turning the gun on himself.
The couple's four-year-old granddaughter escaped unharmed and fled to a neighbour's house to ask for help after she saw the shootings unfold.
The children, whose mother, Jennifer Smalling is believed to be the daughter of the gunman and his wife, were visiting their grandparents when the killings happened.
The couple's neighbours, in Hermitage, Tennessee, said they were stunned by the news.
Ty's uncle, Chase Dodson, has launched a fundraiser to pay for the little boy's funeral. Chase is the brother of Ty's father, who died 15 days before Ty was born.
Writing on Facebook, he said, 'My nephew Ty Dodson was visiting his grandparents on his moms side this past Saturday night in Tennessee. This visit went extremely wrong and the step-grandad murderd (sic) his wife, My nephew, and then cowardly turned the gun on himself. This was the a shock to both sides of Ty’s family.'
Metro Nashville Police Department, who are investigating the deaths, confirmed the details in a Tweet.