The grief-stricken woman told him and his wife that she "flipped off" the driver of a white sedan after he cut her off in the carpool lane, he told the Orange County Register.
"It was an isolated road rage incident between the mum and another driver from a white sedan", California Highway Patrol Officer Florentino Olivera told Los Angeles news station KTLA-TV.
"There was road rage on the freeway and someone pulled out a gun and shot my little brother in the stomach, and he said, 'Mummy, my tummy hurts, '" Alexis Cloonan, Aiden's sister, said, according to KABC.
'Unfortunately, the child that was seated in the right rear seat was struck, ' he said.
The gunman who fired the shot that killed the child remained at large as of Friday afternoon.
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The child was sitting in a booster seat in the back seat of the vehicle traveling northbound on the 55 Freeway, said CHP Officer Florentino Olivera at a news conference on Friday.
The boy was struck from behind when a single round passed through the trunk of the vehicle, the Los Angeles Times reported. Witnesses reported hearing a gun shot from a white sedan right before the child's mother pulled her vehicle over to the shoulder.
"It went through the boy's back", he said.
Paramedics rushed the injured child to nearby Children's Hospital, authorities said.
"We don't have the details of what transpired", he said. He was being driven to school by his mother when the shooting happened.
"Even if they don't have video but maybe saw something that might be related, we would love that information, even if they don't think it's important", he said.
The freeway was shut down for hours as about a dozen investigators slowly walked the lanes looking for evidence.