I don't think anyone would come on here and say "All HS coaches are good guys." We all know that's simply not he case even though we all surely wish it was. The same coach who in one person's eyes is the end all to be all is a total failure in another person's eyes. If your son is playing and doing well the coach is all good. If your son is not playing the coach is a clown. Does that happen? Of course it does.
I think it's the right thing to do to be totally honest with your HS player. If your son is not playing do you know why? Does he know why? Are you both honest with each other? If your son is not playing and you can be totally objective and you have all the information needed and your son is being totally honest and he should be playing but is not how do you handle that?
Me personally "Suck it up. Keep working. Yeah he might be wrong but don't make him right. Keep pushing and keep grinding. Prepare for your opportunity don't wait for it. These situations only prepare you for success down the road. Man up and keeping fighting."
I have never seen a negative person who blames others for his failures change his lot. A negative seed yields a negative crop. On the other hand I have seen what a positive person who takes total ownership in his lot can do. It is a very powerful force that simply will not be denied in the end. A positive seed will always yield a positive crop. You just don't get to choose when the harvest is. Just know that you will have your day and you will be a better person for it.
Don't get caught looking at the moment as the final result of the journey. Sometimes the rocky road with the pot holes turns out to be the road that led to the ultimate success.