Be the squeaky wheel when it comes to your kids, especially when it comes to school teams. I have coached almost all levels and ran organizations. I never saw this coming. We did everything right, and found a star. Only to have a crap coach ruin him. Rant over. Grr.
I am so torn on this approach. I have been on both sides of this. I coached varsity high school for 15 years. Had plenty of parents be the squeaky wheel and generally it isn't a good interaction because the parents are typically (and rightfully so) too emotional when it comes to their kids well being. The problem is many times (not all) the kid isn't working hard or has some other thing holding them back that the parents just can't see. Now I am not saying it's this way 100% of the time but I would say it's close to 95% of the time.
From a coaches perspective it is always better for the kid to approach the coach and advocate for themselves. The coach can point out things the kid will know himself or give specific things for the kid to do to get more opportunity. Parents coming in complaining rarely have all the information to discuss the situation properly. As an example, I had one parent come to me complaining about their kids play time. The kid was "better" than other players playing over him when you talk about sheer playing ability. However, what the parent didn't see was how lazy, disrespectful, disruptive their kid was at practice. When I told the parent that he needed to see what his kid was like at practice to understand where I was coming from he decided to show up at the next practice. However, he sat in the middle of the bleachers for all to see. It was the best practice that kid had in all his years in the program. After practice the parent came up to me to discuss what he saw. I promptly invited him to come to all the practices because that was the best his kid had every been. I told him if he wanted to see what he was really like he needed to hide and not let his son know he was coming. I don't know if he ever came back to practice but the next day his kid was back to his usual shenanigans.
There are proper ways for a parent to approach a coach. I mean you do need to help your kid where you can but part of that is helping your kid understand how to advocate for themselves. If a kid goes to a coach first and still doesn't understand why he isn't playing or what the coach is conveying then the parent should schedule a meeting with the coach and player together. Not afrer a game out of the blue. Actually schedule an appointment. But be ready to hear some things you may not believe about your kid. And bring an open mind to accept these things. The point of the meeting is to outline a path for your kid to earn more playing time, starting spot, etc. Then stay on your kid to do those things.
Now, there are bad coaches and sometimes kids do get screwed. Sounds like that is your situation. In those cases it is very frustrating and very difficult to deal with. You still need to follow the proper path of kid first, then parent/kid/coach meeting. If that still doesn't do it and you as a parent have given it an unbiased look then it is time to go to the athletic director for a sit down with the coach, AD, kid, and parent. This generally won't do much though as the AD is typically going to side with the coach (as they should most of the time) but if there is merit and you have a good AD the coach will be reprimanded and there should be changes. Sucks for you and your kid for sure. But that is the process.
There are bad coaches and it's unfortunate when it hurts you. I feel so bad for your situation. It is so terrible because the time is short and it goes by so quick. Once it's lost you can't get it back. Just heartbreaking.