The coach must have one redeeming quality eh? - he picked your son as an 8th grader and you seemed like you were thrilled by that. If he was such a bad guy, why not extricate yourselves from the situation then?
You seem more politically connected than most (all?) I've seen post here. You seem to know what every other coach and former player thinks of this coach and have no qualms about swapping stories about this coach - behind his back no less.
Perhaps you have poisoned the well with your son. If you feel you have been treated so "egregiously," I cannot see how that does not trickle down to your son.
Let your son stand on his own two feet and fight his own battles. The best way to win over the coach is keep the mouth closed and be the first to arrive and the last to leave. Have your son use deeds and not dialog to make his point with the coach. Your son cannot control a coach's decision. All he can control is his own attitude and effort. I think you've made it eminently harder for your son to learn that by playing politics behind the scenes and not just letting things happen as they ought to.