Ashya King's brother Naveed said the five-year-old had been diagnosed with a brain tumour and was undergoing emergency surgery in a video posted on YouTube last month.
Speaking into the camera in a message directly to Ashya, he said:
I haven't slept. I've been awake all night worrying. Everyone is praying for you. We just want to see your smile again.
No kid at the age of five deserves to have a brain tumour. Let's just hope the doctors know what they're doing and they know exactly where to operate and what to take out, and they take everything out and you can be better.
And when we look back in 10 years' time when you're 15, we can actually see that things have changed for the better. Just because they're bad at one point doesn't mean they'll always be bad.
I love you so much. I can't wait to see you.
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.