The youngster was walking with his mother along the coast path at East Beach in West Bay when he fell over the edge.
He landed on the shingle beach 30ft below.
Coastguards and Dorset Ambulance were alerted to the accident on Saturday afternoon and the boy was rushed to Dorset County Hospital by Portland Coastguard helicopter.
The youngster is believed to have suffered spinal injuries.
Two hours later the helicopter flew to rescue a person who had tumbled 50ft into a ravine in Weston-super-Mare, Somerset.
The aircraft was stood down before it arrived and diverted to the Brecon Beacons where a six-year-old girl and her father were suffering from multiple injuries after also falling down a ravine.
The pair were winched into the aircraft and transferred to Swansea Hospital.
It is believed the child had fallen into the ravine and her father was injured while attempting to help her.
Portland Coastguard helicopter had been contacted because it was the nearest rescue aircraft to the accident.
Portland Coastguard watch manager Ros Evans said: "This was a tricky rescue for the helicopter crew in an area far from their normal area of operation.
"This rescue emphasises the co-operation between multi-service search and rescue organisations."
Lifeguards rescued a kitesurfer who had got into difficulties in Portland Harbour yesterday.
The surfer was taken to safety suffering from exhaustion and his equipment was lost in the water.
A three-year-old boy was found on East Beach in Lyme Regis yesterday afternoon. Coastguards found his parents and the family were reunited.
The driver of a speedboat was cautioned by Weymouth harbour master after his vessel was reported to have towed a ring through windsurfers at Overcombe in Weymouth Bay.
Two crew aboard the yacht Tonic Relief reported engine failure near Abbotsbury and Weymouth's all-weather lifeboat towed the vessel ashore.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article