The child was in a blue Peugeot 208 which collided with a blue Nissan Qashqai on the A99 at Occumster, Caithness at around 4.45pm on Sunday.
He was taken to the Royal Hospital for Children in Glasgow for treatment.
The 25-year-old woman driving and her female passenger, 26, were taken to Aberdeen Royal Infirmary with serious injuries.
The 44-year-old woman driving the Nissan was taken to Raigmore Hospital in Inverness with serious injuries.
Police Scotland said the road was closed for around eight hours for a collision investigation.
Police Sergeant David Miller said: "A young child has sustained life-threatening injuries as a result of this crash with three women sustaining serious injuries and we are appealing for information to establish the full circumstances surrounding the incident.
"I would ask if you were driving in the area at the time and may have seen either car prior to the crash that you come forward and speak to officers.
"If there are any motorists with dashcam footage that were in the area around the time of the incident, please get in touch with police."
Anyone with information on the crash is asked to contact police via 101, quoting incident number 2828 of August 22.