Ashya King's dad Brett claims British authorities threatened to take their son away from them for 11 years after he challenged doctors over his treatment.
He said he and his wife Naghemeh fled abroad with Ashya after being warned he would be taken into care until he was 16 with no contact with his parents or siblings.
Brett made his claims as Ashya made his first visit to the Prague clinic where next week he will start proton therapy to treat a brain tumour.
He has had MRI and CT scans, which discovered the tumour hadn't grown any bigger.
Doctors at the Czech clinic say treatment will give Ashya a 70 per cent chance of survival.
His father, Brett, said Ashya was a 'bit traumatised' after the prolonged legal battle that saw him taken from Southampton to Spain before arriving in Prague on Monday.
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British doctors opposed the plan to take Ashya to the Prague clinic, saying it was not suitable for his condition, but the family disagreed and sparked an international manhunt when they removed him from hospital without permission and took him to the continent.
Brett said: "They said to me if I question anything and ask questions as a father...they would take my son away from me, not just for the treatment but until he was 16.
"I wouldn't be able to see my son for 11 years. I said to my wife this is something so serious that our son could be taken away until he is 16.
"Eleven years without us, he wouldn't know his parents, his brothers, his sisters or anyone.
"We knew we couldn't let that happen.
"We couldn't question them any more. We couldn't let them know our feelings because one mistake on our side and they'd take him away."
Ashya hit the headlines when his parents removed him from Southampton General Hospital against the advice of doctors, leading to an international manhunt that saw the Kings imprisoned in Madrid for 72 hours.
The couple were released amid a public outcry, while Ashya received treatment in Malaga's Materno Infantil hospital.
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