Ashya's father Brett King described his incarceration as "torture" for the five-year-old.
Mr King and his wife were in jail for five days while Ashya was in Spain, but he said their discomfort was nothing compared to anyone who had cancer.
Mr King said Ashya is still traumatised after the enforced separation: "Every time I say goodbye to him, he starts to cry: 'will I see you tomorrow?' sort of thing," he added.
Mr King also told media waiting outside the Proton Treatment Center in Prague that he is concerned about what authorities will say when his other children do not attend school in the UK.
Most of the King family remains in Spain.
Ashya King's parents told the Daily Mail they fled the UK after being told NHS treatments would leave him with life-long disabilities.
The head of the police force which applied for a European Arrest warrant against the parents of Ashya King has now urged their release.
A British mother whose son's life was saved by proton therapy has told ITV that she defied NHS doctors' advice to pursue the treatment.