Robin-John Gibb, known as R.J., and his girlfriend Megan Golub are expecting their first child in July, and the family is convinced the singer would have been delighted at the news.
R.J., who worked with his father on the Bee Gees star's final musical project, classical piece Titanic Requiem, tells British magazine Hello!: "Losing my father was devastating. But now that I'm about to become a father myself, my sadness has made way for a sense of happiness I could never have imagined.
"Dad loved children and the idea of having grandchildren. He would have been over the moon to see his first grandchild. It's a shame that he won't be there to hold him. The doctors say it's 80 per cent likely to be a boy and I know he'd have loved a grandson."
R.J.'s mother, Gibb's widow Dwina, adds: "He loved them. Grandchildren would have found him hilarious because he had a great sense of humour and made everything fun."