Police have begun a nationwide search for a mother who is understood to have run away with her son to prevent him receiving treatment for cancer.
Sally Roberts, also known as Sally Leese, and Neon Roberts are believed to have left the Tiverton area on Monday.
Doctors say Neon requires urgent medical attention having recently undergone surgery.
The seven-year-old boy is the subject of a High Court battle between his mother and father, Ben Roberts, over whether he should continue radiotherapy.
Mr. Roberts agrees with doctors that Neon should continue treatment.
At a hearing on Monday, Mr. Justice Hogg relaxed reporting restrictions in agreement with NHS and Devon County Council representatives to assist the hunt for the pair.
The whereabouts of Neon and Miss Roberts is currently unknown, but officers say they were seen in Sussex and Kent areas on Wednesday.
Assistant Chief Constable Chris Boarland said: "Neon is seriously ill and we have huge concern for his welfare.
"He is currently subject to a Family Court order and I would ask his mother to think about her son’s welfare and contact police and medical services as soon as possible.
"Neon needs urgent medical attention and his well being along with his mother is our prime concern".
Anyone with information is asked to call 0800 0560154. Devon and Cornwall Police is encouraging the use of the #FindNeon hashtag on Twitter.