William’s grandmother – who cannot be named for legal reasons – was one of the last people to see her grandson alive before he went missing from her backyard in Kendall in September 2014.
William was famously last seen wearing a Spider-man costume and is widely believed to have been abducted.
Several search attempts and a long-running police investigation have failed to locate William.
An 18-month inquest into his disappearance concluded in October last year.
Deputy State Coroner Harriet Grahame is expected to hand down her findings in June.
There is currently a $1 million reward for any information that leads to the mystery being solved.
Speaking on 2GB’s Ben Fordham Live this morning, former police officer and lead detective on the case Gary Jubelin said William’s grandmother wished she knew more before she died.
“It would have given her a great deal of comfort before her passing if she had some answers on what happened to William,” he said.
“It’s such a sad case, that one, that’s really touched the heart of the nation.
“Police will never give up on a disappearance like that and there’s always the chance that a small piece of information that comes in could help.”
Mr Jubelin was convicted and fined last year for illegally recording a one-time person of interest in the case.
He maintains that he was only trying to get to the bottom of the case.
He is no longer a police officer.