I agree it was the wrong approach to assume it was his position to lose. My view coincides almost exactly with RJM...and justbaseball!!! Not only do most players have to earn their spot every year, more than a few of those are earning their spot game to game.
What about your son approaching his position coach and/or the hitting coach? The HC is making those playing time decisions for a reason. He might not be willing to discuss playing time at all with your son. He might however be willing to talk about what your son needs to do to get in the line up, or what he is not doing which is keeping him out of the lineup, but I still think the hitting or position coach would be the place to start, if your son truly is in the dark.