The 48-year-old took the child into the bathroom of his aunt's Sydney apartment in May 2023, one-and-a-half hours after learning his partner had made a domestic violence complaint to police about him.
Having to unlock the door of the bathroom with a butter knife, his aunt found the pair lying in a pool of blood in an attempted murder-suicide.
The boy, who was three years and nine months old, could not be revived.
In a phone call from jail after his arrest, the man told his partner he "did himself" first and then asked his son what colour he saw.
The boy said red, he said.
"Do you want to be red too? (He) said 'yes' and then I did it," he told his partner.
The 48-year-old also told prison staff he had killed his son because he wanted them to become two ghosts.
The man - whom AAP has chosen not to name to protect the victim's identity - was jailed for up to 28-and-a-half years in the NSW Supreme Court on Friday after pleading guilty to murder.
Justice Paul McGuire imposed a non-parole period of 18-and-a-half years, expiring on November 30, 2041.
"By killing his own infant, the offender has perversely and egregiously breached the trust, protection and love that (the boy) deserved," he said on Friday.
The boy's biological mother died of natural causes in April 2021.
The father recommenced a relationship with his ex-girlfriend six months later and soon started using methylamphetamine.
The couple had previously split in 2014 over his violent behaviour and drug use.
Days before the murder, he accused his partner of infidelity, smashing her phone and assaulting her in the bathroom of his home.
He also expressed delusions that police were watching him, including thinking two loss prevention officers at a supermarket were police in disguise.
Unable to contact his mother, he abandoned his home west of Sydney, taking his partner and son to his aunt's apartment.
While he was out looking for a place to live, his girlfriend packed her bags and left, calling police to make the domestic violence complaint.
After his arrest while in hospital, he told staff he knew he would be arrested but had no family members to help take care of his son.
He said he tried to kill himself and his son so that the Department of Family and Community Services would not put the child into foster care.
On Friday, Justice McGuire found the man was experiencing bizarre beliefs spurred by paranoia that his son was better off dead than being cared for by someone else.
The man had only made very limited expressions of contrition and remorse.
Recent claims to a forensic psychiatrist that he heard voices telling him to "kill everything he loved" at the time of the murder were inconsistent with statements made to police and hospital staff soon after, the judge noted.
Justice McGuire reduced the non-parole period, however, after finding the man's time in custody would be more onerous because of his mental illness.
He was also likely to spend it in protective custody because of the nature of his crime.
The judge expressed his condolences to the boy's family, noting the impact of the murder.
The child's grandparents described the man's behaviour as horrific and selfish while his sister could not comprehend his cruel and heartless actions.
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