Hit the ball hard. Hit it in the air. If your son's instructor gets him to do both more regularly, you have a good one. If your son does not do this, you may not have a good one.
People in this thread can disagree all they want with the idea that 2 + 2 = 4, but no matter how hard they try, adding 2 + 2 does not equal 5.
Hitting the ball to the pull side in the air hard yields the best results in all of baseball. It's proven. There is no debate here. Increasing contact percentage and ability to adjust also matter. But do not forget what matters most - demolishing baseballs in the air. And line drives are in the air, before anyone complains about fly balls.