Just like a football quarterback calling a different play at the line or in the huddle than what was recommended by the staff. When you stray from the plan, you had better execute.
I had a coach who called every pitch and didn't want you to deviate. I've had another coach who sent in a suggested pitch each time and we had full veto power, but we had to explain why we went against the suggestion when it didn't work out.
Every pitcher is different and there are untold ways to get hitters out, but strategies are often very similar when you boil them down. Whether you are going with the called pitch or what you want to throw, before you start your delivery, you had better believe in that pitch in order to execute it.
Ask yourself this...Is your son just as much of a control freak as the coach and your son's predetermined attitude about the coach's call impacting the execution of the pitches? That's something the two of you need to consider.