The poignant track, called Sydney, looks back at his early life in Australia with his brothers. It was recorded in August 2011, just nine months before his death.
Other songs from the album, called 50 St Cathereine's Drive - the address of the home in which he was born on the Isle of Man - were recorded between 2006 and 2008.
His widow Dwina, who compiled the tracks with son RJ, said: "This fragment of song is poignant, wistful, beautiful and unfinished. He cried when he wrote it, and I wept when I heard it."
The album is due out on September 29.