I am in the same position with a younger guy on my team. Huge arm but all over the place.
First, Sandy Koufax had to take a few MPH off the ball to get control of it.
Make sure his eyes are absolutely locked on the target (e.g. the glove). He could be hitting the batter because he is inadvertently looking at the batter (especially guys crowding the plate).
This suggests that the problem is mental. Lack of focus, lack of concentration, lack of lock on the target, or just plain thinking. He could also be thinking about HOW he is pitching (e.g. his mechanics) versus WHAT he wants to do (e.g. focusing on the target). In my experience, I am only successful when I lock in on the glove and let my body do what it knows how to do.
I have considered having some of my guys take karate or another one of the martial arts to learn focus and concentration.
Is the movement consistent? If not, then that could explain why it's hard to control. He could be cutting the ball slightly differently with each throw.
Is his release point consistent? You can figure this out by taping a series of pictures from the front and make sure he releases the ball at the same point and time with each throw.