Thanks for the info Tooldforthis. I think your son needs to talk to the coach and see if he can move behind the plate. It seems like he would know he's got another one in your son but maybe he doesn't see the catching position as a weakness for the team and that's why he didn't move him.
If your son wants to catch then he needs to catch and seems like only way that's happening is if he talks to the coach. I doubt the coach would take it in a negative way if your son approaches him in the correct way.
I'm betting the kid who was drafted was a late round selection due to his offense and they would find him a position later. I think it's great that you don't question your son to create any negativity. Over the longhaul that is going to help him out tremendously.
Overall I hope your son doesn't have any more knee troubles. I'm walking around on two bad ones and for some reason I don't see them getting much better in the near future. Make sure he does take care of the knees.