East Feliciana Parish District Attorney Samuel D'Aquila said investigators determined the shooting was an accident because the child thought the gun was a toy, CNN reported Tuesday.
Under Louisiana law, a child under 10 cannot be held accountable for a crime.
During a closed court proceeding Monday, the district attorney asked for a court hearing on beginning the process for getting the family needed help.
The family said the boy is getting counseling.
D'Aquila described the boy as "distraught" and "visibly shaken."
The boy was being cared for by his grandmother, Marie Smothers, 87, last Thursday at her home in Slaughter when he shot her.
He had been playing the video game Grand Theft Auto IV when he found a .38-caliber handgun. The boy pointed the gun at his grandmother, who was watching television, and shot her from 3 to 4 feet away, the East Feliciana Parish police department said in a statement.