Hutt Valley Police said post mortem results show he died of pre-existing medical conditions.
An 18-year-old man and a 14-year-old youth have been arrested over the attack and charged with aggravated burglary.
"To say they're responsible for his death is probably not correct, but they were a contributing factor in his demise," Paul said.
But he said he would leave it up to police as to whether they laid any further charges.
"I'm a bit of a forgiving person, maybe too forgiving, so I'm not out on any sort of vigilante-hunt or anything. They have to deal with the consequences of whatever's put to them, but I'm not hell-bent on any sort of retribution or escalated charges."
Paul said he had called his father on Friday evening and he sounded breathless and stressed.
He went to his father's home about 15 minutes later and discovered him dead on the kitchen floor.
On the night of the attack, Mr Morrissey had answered the door to two people who knocked him to the ground with a broom handle.
His funeral will be held on Thursday.
The two alleged attackers will reappear in the Lower Hutt District Court later this month.
A police spokesman said police were not looking at laying any further charges against the two attackers at this point.
- nzherald.co.nz and APNZ
