The mystery boy was found by a member of the public crying near Tower Bridge, south London, on 8 March.
Believed to be 12 or 13, he says he had never been allowed out of his Tottenham home alone and has never attended school.
He also told police he had been living in Tottenham since the age of five and had been intermittently physically abused and tortured.
The boy, who is of North African or Asian origin, said he taught himself English from watching television.
He has now started school and is being looked after by social services, police said.
Police hope that by issuing a photograph they will learn more about the boy's past.
Det Sgt Iain MacPherson said police still did not know whether the boy was released or managed to escape.
He is now doing "very well" settling in to his new life, he said.
Historical cases
There have been a number of historical cases of children being held in captivity, often since birth, and denied contact with the outside world.
In 1970, the authorities in California discovered a 13-year-old girl named Genie who had been tied up in a locked room without contact with the outside world for more than 10 years.
The Genie case had echoes of that of Kaspar Hauser, found wandering in Nurnberg, Germany in 1828.
The boy had apparently been kept confined in a small space within a cellar, fed only on bread and water and denied human contact for all his life.
Police are asking for anyone who knows the boy found at Tower Bridge , his family, or the circumstances of his life should contact the Child Protection Team in Camberwell on 020 7232 6356 or Crimestoppers on 0800 555 111.