Craig Lewis had been threatened with violence by the dealer around the time he assaulted Kieron Barley causing brain damage, the judge said.
Lewis, who is in his early 30s and comes from Birmingham, was given an eight-year jail term by Judge William Davis QC at a hearing at Birmingham Crown Court last month after admitting manslaughter and causing grievous bodily harm. His partner, Louise Barley, who is in her mid-20s, was given a 15-month prison term after admitting child cruelty.
But details of some of the problems Lewis had been facing at the time of Kieron's death in June 2011 have emerged in a ruling published by Mrs Justice Eleanor King following litigation in a civil court.
She had heard evidence at the Family Division of the High Court in Birmingham in early 2013 following the launch of litigation involving Lewis, Barley and a local authority. She delivered a ruling in late March 2013 but that was kept under wraps while criminal proceedings ran their course.
The judge ruled that "on the balance of probabilities" Lewis had assaulted Kieron on May 28, 2011 and June 19, 2011. She also concluded Barley was "guilty of failing to protect" Kieron.
The judge said on June 6, 2011 doctors had revealed that he had cerebral palsy.
The judge said that in addition to the pressure he was under from the drug dealer, Lewis was also having to deal with the "acknowledgement that there was something seriously amiss with" Keiron.