On Wednesday, the 11-year-old's father was convicted of involuntary manslaughter and a felony firearm charge, a prosecutor said.
Curry Bryson will be sentenced on April 20, said Maria Miller, an assistant prosecuting attorney for Wayne County, Michigan.
Bryson was acquitted of second-degree child abuse.
"Bryson was shocked when he heard the verdict," Bryson's attorney, Jerome P. Barney, told CNN. "We didn't expect conviction."
Bryson's son, police said, shot the 3-year-old boy in a car parked behind his father's Detroit residence in August.
Barney told the Detroit News
last year that Bryson was not home at the time of the shooting; two other adults were there, the attorney said. Bryson is a good father who takes care of his son, Barney told the newspaper.
"The facts of the case are ... Curry wasn't at home when this happened," Barney told CNN. "He was at work and got a call (about) what happened."
Prosecutors said last year that the boy took the handgun from his father's bedroom closet, threw it out a window, retrieved it and got in the car.
A short time later, the 3-year-old entered the same vehicle, prosecutors said.
It's unclear what happened after both boys were inside the vehicle, but the younger child was shot once in the face and killed.
"He always had a smile on his face, always smiling, and he loved his sisters," Denishia Walker, mother of 3-year-old Elijah Walker, said in August.
She described the older boy's family as friends of her family.
The 11-year-old initially faced manslaughter and felony firearm charges in connection with the death, but he was cleared of the charges after he was diagnosed as developmentally disabled, Barney said.