Michael Wynn was unable to speak, chew, or swallow, and could not hold up his head due to severe brain injuries he suffered in the 1998 attack. He was left with spastic cerebral palsy and a deformed spine, and died at the age of just 12.
His father, Allan Young, 36, from Wishaw, North Lanarkshire, has admitted shaking him when he would not stop crying, jurors have been told. He was convicted of causing grievous bodily harm on Michael in 1999, but in a landmark case he is now on trial accused of manslaughter following his son's death in 2011.
Dr Richard Ward Newton, a paediatric neurologist at Royal Manchester Children's Hospital, assessed Michael when he was three and found he had almost no control over his own movements.
"He had profound learning difficulties and very little by way of responsiveness," he said. "His functioning was probably less than a newborn baby."
Young denies manslaughter. The trial continues.