I believe it was the divorce rather than the brain tumor that kept these unlovely folks from caring for their severely disturbed son. Grandma obviously snapped, but this was a boy whose parents betrayed him.
The parents pushed off their own responsibility for the boy on a 75 year old. It was no doubt a difficult situation for the teenager and the grandmother.
And being divorced the parents had two possible homes for the boy to live, yet apparently neither wanted him which must have hurt him deeply.
Bingo