Police believe the accident happened when one driver lost control of his vehicle and veered into the opposite lane.
He reportedly cursed at the victim's mother and scolded him with vulgarities.
It had dashed out into the middle of the road.
The items in his luggage were worth around S$3,000.
The victim was only 12 years old.
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The son-in-law has been suspended from the police force since August 2016.
Those in contact with his family or friends have been urged to reach out.
They have also been harassed by loan sharks since their son borrowed money.
The plane was taxiing at a gate at the time.