"Our youngest, Billy, is 4. From the age of about 2 and a half, he has shared several tales that at first seemed silly.
"He provided more information as the years went on and now we don't know what to do with it.
"You see, Billy believes he was once Princess Diana."
The popular TV host, who appears on Today Extra in Australia, said the strange comments began when his wife Lisa received a card with a photo of Lady Di on it.
When Billy saw it, he said: "Look! It's me when I was a princess."
Campbell insists his son has no knowledge of the royals, but the creepiness doesn't end there.
David then overheard Billy referring to his two "boys" and when questioned further the boy, then 3 years old, clarified that he was referring to his "sons".
So far, so bizarre.
Billy then revealed a much deeper knowledge of the Princess' life when he went on to say that he had brothers and sisters including a brother named John.
Campbell and his wife thought Billy must be mistaken but then discovered the little boy was right: Diana's infant brother John died a year before she was born.
The story takes a chilling turn as Campbell tells readers of the moment that "stopped us in our tracks."
"Lisa showed him another photo of Diana, and Billy said: 'There's me as a princess. Then one day the sirens came and I wasn't a princess anymore.' People shiver when we tell them this one."