Our band plays only covers (jazz and rock from the 1930s through the 1960s or so). We get the music online. Occasionally while practicing a new song, our singer/guitar player will diverge considerably from the music and then claim that he's just playing the song as he remembers it. He makes no attempt to change. We are all well aware of recordings of all of these songs that are available online. I feel like saying something like "well, I can't play the song in your head," but I realize that this might not be the best thing to say. I'd rather that the band not self-destruct. Any suggestions ...?