For those suggesting that dad shouldn't have said anything to the coach at the time, but instead should have waited until later, what if his son had been injured throwing another 20 or 30 pitches the next inning?
I agree that all of this should have been discussed before the season, and as a coach myself, I understand that yelling at the coach in front of others isn't the best approach. However, the dad did need to make sure that his son didn't go out for another inning if he didn't feel it was in his best interest. If doing what he did was the only way to get that message across, I would hope that the coach would understand.
Hopefully a parent/coach meeting is in the plans to address this situation moving forward.