Police said five-year-old Ashya King requires urgent medical attention and should be taken to a medical facility as soon as he is located.
Detective Superintendent Dick Pearson, of the Hampshire Major Investigation Team, said:
He needs to be taken to a medical facility for his urgent health requirements as soon as he is located.
We have also launched a social media appeal and would urge everyone to share this appeal, particularly if you have friends and relatives in France and bordering countries.
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.