Your son needs to work through this. Somewhere in the remainder of his baseball experience or adult life there will be a coach/boss change he won't like. This is a life lesson. He can be a bigger person by fighting through it. Let your son deal with it. Don't get involved unless there's physical abuse involved. If that occurs don't approach the coach alone. Make sure other parents agree.
How was the team last year? Did they have players screwing off? Maybe the coach is trying to find out who really wants to be there.
What do you mean by hates select ball and showcases? Is there going to be a price paid for participating or does he just look down on it?
I'm guessing he once heard Augie Garrido made the Longhorns run until they puked after dropping a doubleheader. The story gets grosser but I'll spare you. Sometimes high school coaches who don't know what they are doing try to model themselves after a master rather than be their own coach.