Les B,

Basically, you tell him he's supposed to do it that way because that's they way its done. (or something like that).

That's the way it's done because that's how Maybell or Lester or whoever did it.

The point you are trying to make is that that when he plays backup guitar it is no longer about him, or what he likes, or what he thinks is cool, or what he thinks sounds good.

It is about rhythm. Rock solid rhythm. It is not about being interesting, or pretty, or flashy.

It is about getting into a mindset where playing a song with three whole chords is a treat and playing a song with four is like Christmas (and comes about as often).

My experience with "folkie" guitarists is that it takes them a long to to "get it" about backup guitar. Some never do.