Norman Roberts roped the boy's feet together and hauled him over a beam in a barn at the Anglesey farm where they lived, then beat him, it was claimed at Mold Crown Court.
Mr Roberts' son, Ian, also joined in the beatings, which would happen up to three times a week, it was alleged.
The boy, Paul Llewlyn Jones, was starved by Mr Roberts' wife, Evelyn, and given dog biscuits as a punishment. He resorted to begging from other children at his school and even scoured bins for food, David Hale, for the prosecution, said.
Investigations were carried out by social services at the time and Paul Jones was placed on the 'at risk' register. But, despite that, he was left with the Roberts family, because the social services department considered that they had insufficient justification to remove him.
Paul Jones, now 19, of Holyhead, who was fostered by the Roberts family from the age of six until he was 12, was interviewed by North Wales detectives conducting a major inquiry into alleged abuse in children's homes in the region last year.
He told police what had happened to him at the Roberts' farm at Gwalchmai, Anglesey. Police checked his files at the county council's social services department and found photographs taken at the time, showing he had been beaten.
Mr Hale said that a doctor, who had examined him then, would be called to give evidence, as would a home office pathologist, who believed that the injuries shown on the photographs had been inflicted by a whip.
Norman Roberts, 59, of Ysgubor Esgob, Gwalchmai on Anglesey, denies cruelty. His wife Evelyn, 59, of the same address, denies wilful neglect by allegedly failing to provide the boy with sufficient nourishment. Mr Roberts is also charged with assaulting the child between 1980 and 1985. His son Ian, 34, denies common assault upon the boy in September 1985.
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