A High Court judge says he is gravely concerned about a "vulnerable" 14-year-old boy who has "significant educational needs" and has been missing for a year.
Mr Justice Keehan made a public appeal for help finding Moses John - who is at the centre of family court litigation and has been made the subject of an interim care order.
He said it was "imperative" that the teenager was found.
Police and social services had told Mr Justice Keehan that Moses was thought to be with his mother - Pauline Watson, 46.
Social workers say they have concerns about the way Moses' mother has behaved towards the youngster.
They say she is thought to have mental health difficulties and has spoken of how she "hates" Moses and considers him a disappointment.
Police say Moses and his mother vanished from their home in Northamptonshire in the summer of 2015 and are thought to be in London.
A lawyer representing Northamptonshire County Council - which has responsibility for Moses' welfare - outlined detail at a public hearing in the Family Division of the High Court in London as Mr Justice Keehan asked the public to help find the teenager.
"I am gravely concerned for his welfare," said the judge.
"It is imperative, in his welfare best interests, that his current whereabouts are made known."
He said the teenager was vulnerable and added: "Moses has been missing for very many months. The very best endeavours of the social workers and the police, working co-operatively together, have not succeeded in finding him."
Police say Moses is black and about 5ft 9ins tall. His mother is also black and around 5ft 7ins.
Officers say Moses' mother, who is from Jamaica, is thought to have links to areas of London including Croydon, Islington, Merton, Sutton, Southwark and Lambeth.
Barrister Richard Jones, who represented Northampton council at Monday's hearing, gave Mr Justice Keehan an outline of the background to the case.
Moses had "significant educational needs" - and he said the youngster's mother "might well" have mental health difficulties, said Mr Jones.
He had been made the subject of an interim care order.
Mr Jones told how Moses' mother had spoken of Moses being a disappointment and of how she "hates him".
He also said she was an illegal immigrant - saying her leave to remain in the UK had expired in September.
A detective involved in the investigation said Moses and his mother could be using different names.
Detective Sergeant Nickie Deeks, who works for Northamptonshire Police, urged members of the public to carefully study photographs of the pair.
"Please think carefully because you might have seen them," she said.
"You might have seen them in a shop, in a park, on the Underground, on a bus, anywhere."
She said: "We need to find Moses. If you know where he is, or where his mother is, or you think you might have seen them, please call."