The victim is being treated in hospital after he was struck by the vehicle in Tower Hamlets, east London, on Friday.
He was taken to a major trauma centre by London's Air Ambulance, after the crash in Bow Road, at about 08:15 GMT.
A 25-year-old man was later arrested on suspicion of dangerous driving and failing to stop at the scene of an accident and bailed until February.
The Met Police said on Friday evening that the boy's injuries had become life-threatening and his next of kin were aware.
The car involved in the crash was later found abandoned.
An 11-year-old boy was seriously injured when he was hit by a car whose driver fled the scene.