According to Wikipedia, he acquired both of these nicknames through his association with The Band. On their 1971 album Cahoots, Morrison provided guest vocals on 4% Pantomime, a song he'd written along with the Band's Robbie Robertson. The pianist and lead singer, Richard Manuel, addresses him as the Belfast Cowboy.
At the Band's farewell concert, in 1976, Morrison made a highly-acclaimed guest appearance. As he leaves the stage, Robbie Robertson can be heard (on the live album and in Martin Scorsese's film of the concert) calling out "Van the Man!"
© Haydn Thompson 2020