Anthony Lane Jr., 16, of Eatonville, was playing a video game around 8:22 p.m. Sunday when the child picked up a loaded handgun from the floor and shot Lane once in the head before running out of the room, the Orlando (Fla.) Sentinel reported Monday.
"It was an accident," Eatonville Police Detective Roy Smith said.
It was unclear who owned the gun or why it was on the floor.
Lane was taken to Orlando Regional Medical Center where he was put on life support, the Sentinel said.
Lane's grandmother, Angela Jones, said doctors pronounced him brain dead just before 4 p.m. Monday.
"He's still alive, but just barely," Jones said. "We need prayers for our family."
Jones said the victim did not carry a weapon.
"He is scared of guns," she said. "His mom didn't even let him play with guns when he was little. Not even water guns."
Jones said the victim's mother, whose name has not been released, plans to donate his organs, the Sentinel reported.
The whereabouts of the 6-year-old were not reported.