Diarmuid Frazer and his twin brother were out with one of their parents this week when they were struck by a silver Ford Ka car as they crossed a road in Lisburn town centre.
Both boys were rushed to the Royal Victoria Hospital in west Belfast for treatment to their severe injuries following the collision in Railway Street.
Tragically, Diarmuid lost his fight for life last night in the RVH. His twin brother is still in a stable condition in hospital.
Today, police in Lisburn have asked for witnesses to the road traffic collision to contact them or the Collision Investigation Unit at Steeple Police Station on the non emergency number 101.
The collision occurred at around 3.35pm on Wednesday 26 March 2014.