[philetus]

I see Dennis Quaid plays the boy’s father, but why are they saying that the song is “his tune”?

“Dennis Quaid, one who is known in film, little in song, shares a very adequate performance in his tune “I Can Only Imagine”.

[Albion Wilde]

I see Dennis Quaid plays the boy’s father, but why are they saying that the song is “his tune”?

See post 39. Dennis did not write "I Can Only Imagine", but he did write and performs "On My Way to Heaven", while he was working on the film. There is a link at the bottom of post 39 that tells the whole story.