Speaking in a soft voice, the Hollywood star told a court of his vain efforts to resuscitate the 16-year-old at the family's holiday home in the Bahamas.
He was giving evidence in the capital Nassau at the trial of two people accused of attempting to blackmail him for £15 million over a document relating to his treatment.
Earlier, the grieving actor, a prominent scientologist, had arrived hand-in-hand with wife Kelly Preston, accompanied by supporters from his faith.
He waved as a fan shouted: "We love you John."
Questioned about whether Jett had medical problems he replied: "My son was autistic and had a seizure disorder.
"Every five to ten days he would suffer a seizure lasting 45 seconds."
The court had already heard from a policeman on the scene that John wanted the youngster rushed to an airport where he had a plane waiting to transport him to the US – rather than a local hospital.
Inspector Andrew Wells said one of the paramedics called to the house had wanted a signature on a statement confirming that the screen hero had waived medical treatment for his son.
The Taking Of Pelham 1 2 3 actor said he didn't read the document – known as a refusal of medical attention form - because "time was of the essence".
It is claimed that the paramedic and another defendant demanded money not to make it public.