The 15-year-old, who was driving, suffered serious head and pelvic injuries in the incident on Saturday night.
His two 15-year-old passengers were treated at Crosshouse hospital for "minor injuries" and are due to be released.
It happened at around 10pm on the A713, south of Dalmellington, when the vehicle lost control, left the road and crashed in an embankment.
Police said no other vehicles were involved in the crash.
Anyone with information is asked to contact 101.