the bottom 3rd of the strike zone gets lived in by good pitching at the HS varsity level, I could search the batting averages by zones but I don't need to know the exact numbers to be aware of this. if you are going to be an elite hitter in HS or higher you are going to need to learn to hit the bottom of the effectively. I didn't mean to be to be smart ass but the man asked a simple question.
you son obviously has some power, if he continues with your stated results he is going to get more and more of the offspeed...so he better learn to adapt. I have watched my son learn the same way, until he hurts them something soft that is what he is going to get.
the fact is every batter almost always gets at least one good pitch hit during an at bat...the higher the level you go the fewer of those pitches you get. if you watch replays of counts, even in the majors, there is almost always at least one good pitch to hit.