The parents of cancer-stricken Ashya King said they fled the UK after being told NHS treatments would leave him with "life-long disabilities".
Brett and Naghmeh King said doctors warned them their son could suffer a number of side effects from conventional cancer treatments.
These included secondary tumours, hearing and growth problems and "special needs for the rest of his life".
In an interview with the Daily Mail, Mr King said he asked several times about proton bean therapy after a cancer doctor told him it was "superior" and had fewer side effects.
But he claims another doctor told him: "If you continue with these questions your rights to make decisions about Ashya will be taken away from you."
Mrs King told how Ashya's first operation to remove a brain tumour left him "like a vegetable with his eyes open" and she had to hold his eyelids down so he could sleep.
At one point his suffering seemed so bad she even questioned if it was right to keep him alive.
Faced with the prospect of further health problems for their son, the couple decided to take Ashya to Prague, where proton beam therapy treatment was available. They planned to pay for it by selling their house.
But two days later they were arrested in Spain after doctors called police when they discovered the boy had gone.
They were kept in prison for three days while Ashya was alone in hospital hundreds of miles away.
Public outcry over the couple's treatment later led to them being released without charge.
The NHS has since agreed to fund Ashya's treatment, news which Mr King told the Mail came as a "weight lifted".
A spokesman for NHS England said: "Now that Ashya is in Prague, it is clearly best that Ashya continues to be treated uninterrupted so the NHS has agreed to fund this care, as requested by his parents, in accordance with relevant European cross-border arrangements."