Joseph Smith suffered multiple injuries when the Ford Focus car he was travelling in smashed into oncoming traffic on Cardiff's Western Avenue.
Driver Dean Collins, 24, from St Mellons, denies causing death by dangerous driving in September 2015.
Cardiff Crown Court heard witnesses described his driving moments before as "impatient".
The trial was told that Mr Collins was in a borrowed car carrying his partner Laura Bright, 23, her mother Michelle Holmes, Joseph, and Mr Collins' own two-year-old daughter.
The grey Focus car crossed the central reservation on Western Avenue hitting a white Seat Ibiza head-on.
The jury was told that rear-seat passenger Joseph was wearing a seatbelt, but was not on a booster seat.
Prosecutors said it meant the child was too short, with the seatbelt across his neck.
The boy suffered broken limbs, a fractured spine, head and internal injuries.
"Joseph was not sitting on a booster seat as required by law," said prosecutor Janet McDonald.
"This impacted his chances of survival.
"The prosecution say the responsibility for that child was that of Dean Collins."
The trial heard that Mr Collins' own daughter had been strapped into a booster seat, and survived the crash.
His partner and her mother, and the driver and passenger in the Ibiza car all suffered critical injuries which required "intensive medical care".
The defendant has also pleaded not guilty to four charges of causing serious injury through dangerous driving.
The trial continues.
A five-year-old passenger was killed in a head-on crash because he was not on a booster seat, a trial has been told.