A CHP spokesperson confirmed the boy's death at a news conference. At an evening press conference held by the child's family, the boy was identified as Aiden Leos.
A GoFundMe page was established for Aiden's mother and to help with her child's burial costs.
The fatal incident, described by the CHP as an isolated road-rage confrontation, occurred near Chapman Avenue between 7:55 a.m. and 8:15 a.m.
At the time of the shooting, the victim was sitting in the back seat of a silver Chevrolet sedan as his mother was driving northbound on the 55, said CHP Officer Florentino Olivera.
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The shooter, who remained at large, was said to be driving a newer model white sedan, possibly a Volkswagen wagon sedan, according to CHP. The suspect vehicle was last seen traveling northbound on the 55 from Chapman Avenue. A description of the suspect was not immediately available.
The northbound 55 Freeway was closed for hours at the scene, and video from AIR7 HD showed a phalanx of officers scouring the pavement for evidence.
Speaking to reporters on an empty on-ramp, Olivera issued a plea for witnesses to come forward with any information they may have.
"If your vehicle has a dashcam, we're asking you, please provide that video," he said. "If you were driving by, you saw something that was not right, call it in -- even if it wasn't something major."
Aiden's mother told a relative a male and a female were in the suspect vehicle.
Tips may be provided by calling the CHP's Santa Ana office at (714) 567-6000.
The Highway Patrol emphasized that the incident was not connected to the string of more than 80 reported shootings at vehicles on California freeways since April.
All northbound lanes were reopened on the freeway shortly before noon.
CORRECTION: A previous version of this article erroneously reported that the child was airlifted to a hospital. The victim was transported by ambulance.