The parents were sought by police on child cruelty charges after they took Ashya, who has been treated for a brain tumor, out of a British hospital against doctors' advice. They then traveled to Spain where they planned to sell a property to pay for proton beam radiation therapy in the Czech Republic or the United States.
They were arrested on Saturday and were being held in a prison near Madrid pending a British extradition request, but were released after U.K. authorities dropped the charges against them.
Their ordeal has generated a heated debate over whether authorities acted appropriately in chasing the parents through Europe.
The boy had a brain tumor removed and needs treatment to prevent it from returning.
Brett King told reporters in the Spanish city of Seville earlier Wednesday that everything he and his wife did for their son was for the boy's own good.
"We just want to help our son get through this bad time, because he hasn't got too many months to live," King said.
The British doctors have previously said Ashya's chances of survival were between 70 percent and 80 percent with standard radiotherapy.
King criticized his son's medical care in southern England at Southampton General Hospital, saying "they were going to kill him in England or turn him into a vegetable."
He did not say what his immediate treatment plans are for the child.