Chris Watson said his son was allowed out several times to see his mother in the "couple of months" she was in hospital. He was accompanied by guards.
Scott Watson told his father he wanted to speak at the funeral. "He wants to get up and say his piece."
Corrections has refused to comment. "The department does not comment about the movement of individual prisoners," it said in a statement.
According to the Corrections website, prisoners are sometimes allowed temporary release on compassionate grounds to visit a seriously ill relative or to attend a funeral.
In some other cases, compassionate leave is granted to attend the birth of their child or visit the newborn.
Chris Watson said his son was "bearing up" after his mother's death, as was the rest of the family. "We have a wee whimper every now and then."
He said his wife had been unwell for a couple of months but had been "progressing quite well. Then it turned to garbage quite quick".
After an 11-week trial in 1999, Scott Watson was convicted of murdering Ben Smart and his friend Olivia Hope.
They were last seen during the early hours of January 1, 1998. Their bodies have never been recovered.
Watson was sentenced to life imprisonment with a minimum non-parole period of 17 years.
He has always denied being responsible for the killings and has tried on a number of occasions to have the case reopened and his convictions overturned.
The Court of Appeal rejected an application from him and the Privy Council later found there were no further grounds for appeal.
