Paul was playing on his own when he fell into a lake near his grandparents’ house in the town of Lychen, north of Berlin, Germany.
The child’s grandfather later found him lifeless in the water.
Paul was quickly dragged to the shore but the youngster remained unconscious.
His father, who had had first aid training in the past, tried to resuscitate his son by giving him mouth to mouth and heart massage.
A helicopter took him to Helios hospital in Buch and doctors also tried to resuscitate him but he was unresponsive.
They were about to stop because the boy had been clinically dead for three hours and 18 minutes – but then a miracle happened.
The team managed to get his heart beating again, defying the laws of medicine.
The water in the lake was cold and the boy’s core temperature was just 28C – it should normally be 37C.
If the temperature had been higher, the team would have stopped trying to resuscitate after 40 minutes because the boy would definitely have been brain dead.
Cold temperatures means the metabolism slows so body can survive with little oxygen.
Professor of Paediatrics, Lothar Schweigerer, is from the Helios hospital in Buch.
He told Sky News: “My doctors were close to saying ‘we can do no more’ after two hours of thorax compression.
He said this was “because the chances of survival had gone and the little lad must have been brain dead”.
The professor added: “But then suddenly his heart started to beat again … it was a fantastic miracle.
“I’ve been doing this job for 30 years and have never seen anything like this.
“It goes to show the human body is a very resilient organism and you should never give up. The boy is happy and healthy. It’s a wonderful thing.”
He told daily newspaper Bild: “Paul said to his parents, ‘I was with Oma (granny) Emmi in Heaven. She told me to go back really quickly.'”