Ashya King's father said his son had "lost his spirit for life" during the four emotionally draining days they were apart.
He told reporters Ashya was now "doing worse" than when he was with them.
ITV News reporter Sejal Karia reports from Malaga:
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.