Answer: Dick Cavett
Forrest, after his return from the ping pong championship in China, was a guest along with John Lennon on a talk show where Dick Cavett was the host. Forrest appeared to be a deer staring into the headlights as John commented on Forrest's impression of China using lyrics from one of his songs - "No Possessions?" "No Religion, Too?", "Easy If You Try, Dick" - that was when John Lennon addressed Dick by name.
"Imagine" was the song - Priceless!