Hugh, who was "hopefully" being discharged from hospital today with a fractured skull, "genuinely can't remember anything".
"When he was found he was apparently very cold, so he could have been unconscious for several hours."
Hugh was also left with a brain bleed, a broken eye socket, and possibly a broken collar bone and rib.
Soper said his son originally believed he was assaulted because his cellphone was gone. The phone was later found not far from where Hugh was discovered.
"The first thing he can remember is the flashing lights of the police car and an officer shining a torch at him."
A police spokeswoman said Hugh's condition had improved significantly.
"Inquiries have established his injuries were sustained as a result of a fall down a steep bank while walking home," she said.
"He was alone at the time of his fall, and was not the victim of an assault."
The incident follows an attack in July when Soper's other son, Henry, was stabbed on a night out in Wellington CBD.
Henry, 23, was leaving a bar in Courtenay Place in the early hours of July 28 when he got into an argument with a man he didn't know, who is alleged to have pulled a knife.
Soper said Henry put his arm up to fend off the blow but the knife went through his forearm, slicing nerves, tendons and damaging an artery.
Bleeding heavily, Henry was taken to hospital.