It’s not always evident why coaches make the decisions the make in the moment. It usually visible over time. If it’s that important to know now tell your son to confront the coach. I guessing you will be on the receiving end of, “Are you kidding!”
My son won the starting second base position mid season freshman year. He hit ,300+ and made the all freshman team. He returned the following year to find an All American JuCo at his position, a Gatorade POY freshman in center and the son of a former MLBer at short. He didn’t start the first two games of the season. He got two hits on Sunday. He started the remInder of the year. He made honorable mention all conference.
The following year the coach added another Gatorade POY. Once again after hitting .300+ and making a level of all conference he didn’t start the first two games. He got two hits on Sunday and started the remainder of the season.
It didn’t take long before he was batting second and leading the team in almost everything. He made second team all conference.
How it netted out was my son was capable of playing seven position. The coach never explained why my son didn’t start opening day. He did later say he knew my son could hit and play anywhere.
The point is a player has to roll with the minor stuff and remain physically and mentally ready to contribute. One opportunity may be all it takes. One successful opportunity leads to another. One opportunity may be all a player gets. Freshman year after four pinch running stolen bases and showing good instincts on the bases the coach decided to start my son and see if he could hit his way on.
Hang in there. Stay positive. Don’t pour any of your negativity on your son. There are about twenty pitchers competing to be the ten who contribute. Negativity and lack of mental preparation won’t get him there. All twenty are capable or they wouldn’t be on the team. But it’s not going to end well for about a half of them. It’s just the way it works.