The three areas where you will encounter this issue is pitching, hitting and catching.
As HS doesn't want to hear from the parent, I would say have the kid try first. Unfortunately this is a situation that I think a parent SHOULD get involved if need be. Obviously feelings and egos get involved way to much with coaches. Its not like your athlete is changing up assignments that are used by his other team or anything. Its mechanics. I would appreciate the athlete seeking additional help. And if I were to make a suggestion or request a mechanics change and he says he prefers to do it this way....Take note...And if the coach feels that it is really holding back his accuracy or speed, tell him. If the kid refuses, than its on him and his lack of playing time. But simply because it isnt HIS way? coach is dead wrong.
Ran into this issue catching. My son was playing legion (more so to play with friends during the summer and travel for competition). Among other issues such as overuse, he did not react well to an assistant coach trying to change his mechanics. He tossed out two from the dish and they stole none one game and the next the coach was trying to change his whole approach. T politely advised the coach if he could continue to do it the way his former coach (mlb experienced catcher) has instructed him. The coach yelled at him and sat him for a game. T advised the head coach that he was there more out of courtesy and if the assistant tried that again he would walk.
It only half solved the problem and the assistant tried to find a replacement for T but that experiment was short lived. And since we play enough on the weekends, we wont play legion anymore. This assistant got into it with the head coach of legion and while I was down helping coach MS ball, he also argued with the head middle school coach. some guys...sheesh.
With HS ball...totally different beast. It helps we have some family that are HS coaches and we do occasionally run into the AD from time to time who is always open for a "general" inquiry on how to handle things.....