I will take this opportunity to say I think it is tougher for us as coaches/parents. I was an assistant coach for varsity baseball and football and head middle school coach when my kids were younger. I think it has somewhat changed my perspective as a parent for the good and the bad. At times, I relate to the choices that my sons' coaches had to make. But I also see the bad choices they make in relationship with my son. I have had coaches that didn't play to win like I do. They played everybody to some extent, even on varsity, and gave in to requests from supporters and administration. They played kids that had no business playing but were good for the program or school. They played seniors who shouldn't have been on the team much less play varsity. I have been encouraged to do so by administration but have never given in to that because for one my livlihood was not dependent on that job. I did it for the love of the game and to win.
I have never completely felt like someone held a grudge against my kids but I have experienced the coaches who, because of our relationship and my sons good nature, felt like they could bench them for others who were not as good and "needed playing time." They were right. I never said anything but really had to bite my tongue a few times.
This situation does sound familiar though in lots of areas of life. I think there is a lot more of this in school ball than we would think. But let me clarify one thing that has stood out to me. The OP says his son is not the best. I'm willing to bet that the coach would do otherwise if the OP's son was the stud. Because he is a middle level player it is more acceptable. He stated that he was 7 or 8 out of 11 so that position would be substituted a lot. Even in baseball, I substituted my 6-8 batters and never my 1-5 no matter how they were hitting that day, unless it was just horrible. I also can guarantee i would never bench my son if he was the stud to give a disgruntled parent the benefit of their kid getting playing time. I would say this is a very prevalent thing in sports today. Adults/parents not getting along. I have a kid I do lessons for that went to another league because the dad and the other coach/dad did not get along. The kids were the ones punished.