He is being treated for a serious head injury at Kings College Hospital in London, police have said.
A 33-year-old man from Polegate is also being treated for head injuries and a broken arm and leg.
A BMW 5 series car, which was travelling westbound on the A259 in Hooe, East Sussex, collided with trees and signage shortly before 2.45pm yesterday.
Police are now appealing for witnesses to the collision.
A spokesman said: “Police are appealing for witnesses to a collision on the A259 at Hooe which left the driver and a passenger with serious injuries.
“A BMW 5 series car travelling westbound between Bexhill and Eastbound on the Marsh Road at Hooe left the road between the junction of Green Lane and B2095 and collided with trees and signage shortly before 2.45pm on Friday.
“The driver, a 33-year-old man from Polegate, sustained head injuries, a broken arm and a broken leg and is being treated at the Royal Sussex County Hospital, Brighton.
“A seven-old boy, who was a passenger, suffered a serious head injury and is being treated at Kings College Hospital, London. His condition is described as critical.
“The front seat passenger a 25-year-old woman from Polegate sustained slight injuries.”
Sergeant Huw Watts added: “We would like to hear from anyone who either witnessed the collision or who may have seen the vehicle being driven prior to the collision. We would also like to thank drivers for their patience while the road was closed for five hours while we dealt with the incident.”
Any witnesses are asked to contact police by reporting online or by calling 101 quoting Operation Lakeshore.