Mental is always part of it. Confidence is important and confidence is a product of mastery.
Try to get him to recognize the type of pitch and where it's going to end better. It's called pitch recognition. Not hit the green ball don't hit the red or look for the number on the ball drills. This is actually seeing the shape of the pitch and the initial trajectory factors.
Dr. Peter Fadde is the best mind in the game at this skill.