The judge said that the experience of being locked in a windowless room in "appalling conditions" had taken a physical and psychological toll on the child victim.
Doctors who examined the boy said he was underweight and below average height for his age, and treated him for anaemia.
But Judge Norman Wright said that while the physical effects have been remedied, the psychological harm "will be unknown".
"It is bound, in my judgment, to be profound," he said.
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