The boy, identified as Junior Lam, a Year Five pupil of SJKC Yuk Yin, was one of four passengers in the Toyota Hillux vehicle driven by his 24-year-old relative.
The other three passengers are also aged 11 years.
District Police Chief DSP Robert Salisip @ David said the driver lost control of the vehicle while trying to avoid hitting a buffalo, resulting in the vehicle swerving to the right and hit a pile of sand before overturning in the middle of the road.
He said the victim, who was sitting on the rear passenger seat, sustained severe head injuries.
The driver and other passengers sustained minor injuries. The case is being investigated under Section 41(1) of the Road Transport Act 1987.