"I'm so happy, for one month I've tried to get here and now we've arrived, it's amazing," Ashya's father Brett King told reporters after landing in the Czech capital where his son is scheduled to undergo alternative therapy.
"When you see my son, you'll see it's worthwhile," he said.
Ashya appeared fatigued while attendants loaded him into an ambulance.
He underwent surgery for a brain tumour in his native Britain before his parents removed him from hospital and is now being prepared for proton beam therapy in Prague, a treatment that was unavailable to him at home.
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"He's very good, he's got his sense of humour back," King told reporters. He said Ashya would undergo scans Tuesday or Wednesday.
Earlier today, Czech doctors expressed concern that Ashya's parents, who are Jehovah's Witnesses, might object to a blood transfusion that he needs.
A British judge today formally lifted Ashya's ward of court status in London. Judge Jeremy Baker also described the treatment proposed by parents Brett and Naghmeh as "perfectly reasonable".
Peter Wilson, lead paediatrician at Southampton General Hospital in southeast England where Ashya was treated for his tumour, did not believe the alternative therapy would offer any benefit over radiotherapy treatment.
The boy has been in the middle of a legal saga that began when his desperate parents took him out of the Southampton hospital against doctors' advice on August 28.
The case received widespread British coverage in the British media, with public opinion shifting from outrage to sympathy for his parents.