First of all welcome to the site. You are in a tough spot. So maybe I can offer you some words of encouragement. Lets talk about you first. You are hurting for your son because you want him to get what he wants to get. When our children want something we want to help them get it. When they are upset we are upset. And when we feel there is nothing we can do to help them, or we feel like we just don't know what to do that is not only frustrating it's painful.
What your son needs to know is that no matter what goes on on that field you love him just the same. That no matter if he plays a little, a lot, or not at all that has no bearing on how proud of him you are. Encourage him to continue to work hard. To go to practice each day excited about the opportunity to get better and to prove the coach wrong. Encourage him to keep a good attitude and not to sit around waiting for an opportunity but work to be prepared for it.
Stay positive he is tougher than you think. He will handle it much better than you. He will handle it better if you handle it well. The worse thing that can happen if you take the positive approach is that he doesn't get much PT this year. But he keeps a positive attitude and goes into the summer hungry to play and work even harder than he has before. You can use this as an opportunity to show him that when adversity strikes you hold fast. You stay positive and look for ways to turn it into a positive. Sometimes the things we think are horrible are the things that drive us to where we can go.
Let this build a fire under your son. Use it to build a fire under your son. Don't go the other way with it. When someone tells you your not good enough you have choices to make. Prove them right or prove them wrong. Use it as motivation or use it as an excuse to become bitter and have a bad attitude. Let your son continue to do what he is doing. Encourage him. Tell him to keep fighting. Tough times don't last. Tough people do. Good luck. Baseball is not a sprint. It's a marathon. Run the full race. Don't let today keep you from having a great tomorrow. Again, good luck.