The youngster, known only as Kanan, was with foster mother Marcia Langton, 46, who has been arrested on suspicion of abduction and last night remained in police custody for questioning.
Her husband, Darren Langton, is still being sought by police after the couple went missing on Tuesday. They had failed to turn up with the boy to a meeting with social workers for Newham Council, in London, which was taking legal action to return him to its care.
Mr Langton, 40, was recently fined for an assault on a neighbour in Doncaster and is understood to be facing a second court case in connection to a neighbour dispute. He had been bailed to an address in Skegness from where the couple disappeared.
Newham Council said "serious concerns" had prompted it to seek Kanan's return after he was with the Langtons for 16 months.
It is not known if the court cases prompted the concerns but the couple had also reported an alleged campaign of racial harassment to Doncaster Council earlier this year. They alleged this included harassment from the two brothers who went on to commit a savage attack on two boys in Edlington last year and who lived very near the couple in a different area of the town.
Doncaster Council, which has been the subject of government intervention after a series of failures in its care of vulnerable children, did not respond to questions about what involvement it had in the Langtons' case.