Judy:
Best of luck. I hope it works out. There are no easy answers. I believe high school baseball matters a lot, every bit as much as summer ball and in most cases more. At least that is the situation as I have experienced it in Arizona. In fact, usually (but not always) it takes a strong high school performance to create summer opportunities with the better teams.
Most coaches do not like to see private coaches during the season. This is because the player can get conflicting information and it leads to confusion. If you are going to use a private coach, it is probably best if you keep it quiet.
Perhaps your high school coach is good, perhaps not, but I have never seen a coach who did not what to win. And to win, you have to play your best players. Pitchers who do not throw hard have a lot to overcome nearly everywhere. Their margin of error is small. You are undoubtedly correct in your comments about trust. I have never known a coach who does not go first with the players he trusts. Trust almost always is earned in practice and confirmed in games. All your son can do is his best and then hope that it leads to greater opportunity.
Keep in mind, too, that no matter what the coach decides someone will be unhappy because there are more players than positions. The one thing that never pays off with coaches -- good and bad -- is to compare players. The more your son can work within his coach's system the better off he probably will be.