The boy, Aiden Leos, was shot and killed last week along the 55 Freeway in Orange.
The victim's family first offered a $50,000 reward before Orange County Supervisor Don Wagner approved an additional $50,000 to be added to their reward from the county's Board of Supervisors.
According to local broadcaster KTLA, fellow Supervisor Katrina Foley matched the $50,000 with her own $50,000, bringing the total to $150,000.
“It is my hope this adds 150,000 reasons to encourage the public to send in any tips and information they might have, directly leading to the arrests of the individuals involved in Aiden’s death,” Ms Foley said in a statement.
Later, an additional donor added another $50,000 to the pot, raising the reward to its current level.
The child was killed last Friday while he was being driven to kindergarten by his mother, Joanna Cloonan.
Ms Cloonan recalled a vehicle, which she remembers as possibly a white Volkswagen station wagon, cutting her off in the 55 Freeway's carpool lane. According to family members, Ms Cloonan made a gesture at the car for cutting her off.
"As I started to merge away from them, I heard a really loud noise," she told ABC News. "And my son said 'Ow,' and I had to pull over. And he got shot."
Ms Cloonan called 911 as soon as she realized her son had been shot.
The boy was rushed to the hospital but was later pronounced dead.
“He was beautiful and he was kind and he was precious, and you killed him for no reason,” Ms Cloonan said. “I want to find them and I want there to be justice to be served for my son.”
Mourners have built a makeshift memorial for the child on the freeway shoulder near the spot where he died.
Visitors have left cards, balloons, toys and have lit candles to remember the child.
The California Highway Patrol is asking anyone who may have been in the area of the shooting between 7:15am and 8:15am on 21 May to check their dashcam or cellphone videos to any information that could help them identify a suspect.
A GoFundMe account was established for the family to raise money for the reward has already reached $225,000.