If it were my son, I would tell him to make the most out of each opportunity in the field and at the plate no matter what position he is playing.
I'm positive that the coach intended to give him every chance at position X, but when he left for camp and the kid that played in position X in his absence played the field just as well and probably batted better than .099 should be given the same opportunity?
The coach must think highly of your son to keep playing him at Y and Z in hopes that his bat does come around.
Be thankful he is getting the playing time, even if its not the position he wants to play. It will make him a better well rounded player and person in the long run.