The younger brother of one-punch victim Thomas Kelly was targeted with hazing and death threats, leaving the teen broken months before he took his own life, his parents say.

Speaking with 60 Minutes on Sunday night Ralph Kelly said abuse directed at his family over lockout laws weighed on his son, Stuart, who suicided in 2016.

"It took its toll in the end ... We paid the price very badly for it," Mr Kelly said.

The teen's parents also said he became a different person overnight after attending St Paul's College at Sydney University.