Jeremiah Lee Wright told cops "that he'd gotten to the point where he was tired of taking care" of his son Jori Lirette, who had cerebral palsy and heart problems, needed a feeding tube and was in a wheelchair.
So he grabbed the butcher's knife and cut up his boy over the kitchen sink at their Thibodaux, LA home before dumping his head outside for his girlfriend Jesslyn Lirette to find.
His hands and feet were discovered in garbage bags nearby. She told the Daily Comet that the boy had loved his father more than anything but he "killed him anyway."
She said: "I never struggled with my son. He was happy. Everything I could do, I did Jori was my insight, my spirit. My child is gone, and I cannot replace him. He didn't deserve to die."
She added that she had planned on ending the relationship for some time but "I didn't get out fast enough."
Thibodaux police chief Scot Silverii called the crime "heinous" saying he had never come across a crime so bad in his career.
He said: "Not in 22 years have I seen something like this. It's just heinous. He said when he put his head out by the side of the road it was so the mother would see it when she came by.
Wright, 30, was arrested on first-degree murder charges Sunday and held in lieu of $5 million bond after he confessed to killing of his son.
If convicted, he faces mandatory life in prison and is eligible for the death penalty.