Does he step out during the pitch? If so, it sounds like he's been hit before and may be afraid of the ball. You need to work with him and make sure that he knows that it's important for him to always stay in the box and teach him to get out of the way.
You should show your kid his favorite big league baseball player, many of them have been raised through the college baseball system where the rules state that a player must keep one foot in the box at all time...as such many major big leaguers follow this rule out of habit.
The batter's box is the defined place that a hitter must be in to legally make an attempt to hit the ball. It may be helpful to shoe your kid how difficult it may be to reach the ball if too far away, or how dangerous it could be to be too close to the plate or to the catcher.
To keep your kid from stepping out of the batter's box it is best to pitch to him yourself to increase his confidence when batting. He is most likely stepping out because he is afraid of the ball so confidence will help.
Remember that getting hit by the ball is never a pleasant experience. Especially for children. While plate discipline is necessary, it is something that will be learned, but not taught.
The most important thing is positioning. Positioning is knowing where your feet are, and batting stance. The more you keep your feet in position, and have good stance, the better.
Try throwing tennis balls for awhile. If struck with an inside pitch they hurt alot less. May I also suggest throwing BP to the outside part of the plate and gradually working him inside. That way he is less likely to get hit.
