Please don't act like these HS coaches are gods. For goodness sakes, they are human and make 99 mistakes a day with respect to baseball decisions. They favor some kids over others for opportunities on a whim--on nothing more than personality or style. Remember, there are 5% superstar players that no coach can deny (and won't, even if these players are arrogant twits). There are bottom 20% who simply don't make the grade. The 75% in between are of equal or about equal talent levels. That crunch is where the coach decides, plain and simple, which kids to favor with the opportunities to develop. This man's son has fallen out of favor based upon some prejudice, or personality thing, or style thing, or because he did not suck up well enough. Let's be honest here--among that 75% in the crunch, the coach cannot distinguish based on talent so he picks his favorites and plays them all the time and we end up with a self-fulfilling prophecy. The kids who get the reps grow into players. The kids relegated to the bench don't. Reverse it and see who succeeds.
This is not to say that this does not reflect life. This same breakout occurs on our jobs all the time. Some career superstars are easy to spot right from the start. Others fall by the wayside quite quickly. The 75% in the crunch are hard workers who scratch and claw to climb to the top with the 5% guys.
Heck, the same thing applied out there in the jungle. 5% did well no matter what. 20% failed and were eaten alive. 75% did all they could to survive, propogate, and feed the offspring. This is all the nature of life.
Now, that said, if you want your son to look at this in focus, tell him to stick to his guns, bust his tail, work harder than anyone else, all year round, get bigger, faster, and stronger, develop skills, always be the most positive kid out there--and he will make it. He will break down this coach's barriers through sheer determination and force of will. If he does not want to go that route, tell him you can find him a job pushing a broom or mowing yards or cleaning toilets--he can spend the three hours or more he would normally spend on baseball each day working. That may be a dose of reality for him that he needs.