Orange County sheriff's homicide inspector Dan Salcedo said the unidentified boy was found slumped over his wheelchair Friday afternoon on a Capistrano Unified School District bus in Aliso Viejo.
The boy was discovered by another disabled child's mother who had gotten on board to bring her child off the bus.
The mother and the bus driver dialed 911 and tried to resuscitate the boy. He was then transported by paramedics to Saddleback Memorial Medical Center in Laguna Hills, where he was later pronounced dead.
Salcedo said no foul play or criminal activity is suspected in the case. The boy was attending a Dana Point special needs school.
The inspector also said there was no apparent illness involved in the boy's death and his death was unexpected.
An autopsy was scheduled for Monday.
City News Service contributed to this report.