The unidentified boy was rushed to Children's Hospital Orange County but did not survive.
It's unknown how many rounds were fired and the only description of the suspect vehicle is a white sedan, DeMatteo said.
Valdivia, who said he served in the US military, said seeing a child shot was especially hard to take.
'Unfortunately, the child that was seated in the right rear seat was struck, ' he said.
The incident happened around 8 a.m., while the child's mother was driving a Chevrolet silver sedan northbound along the 55 Freeway in Orange, California Highway Patrol said. "We're not sure how many rounds were fired, but a shot was sacked into the auto which struck the child", CHP Officer Florentino Olivera told the Daily News. He said the driver of the white sedan shot into the back of the mother's Chevy Cruze sedan.
The boy was struck from behind when a single round passed through the trunk of the auto, the Los Angeles Times reported.
Other motorists stopped to help, including an off-duty police officer who performed CPR on the victim until paramedics arrived.
"Please help us find the people that did this to my little brother", his sister, Alexis Cloonan, said at the press conference.
Reyes Valdivia and his wife had entered onto the freeway having already dropped off their two children at school when they spotted the mother's auto frozen on the shoulder.
CHP officials said the shooting was not related to dozens of recent BB or pellet gun shootings on the 91 freeway in Orange, Riverside and Los Angeles counties. "If you were driving by, you saw something that was not right, call it in - even if it wasn't something major - call our office".
The 55 freeway was shut down for hours as investigators combed the lanes for possible evidence.