So, I know Derek, and I don't think he's a bad guy, but I have no real opinion beyond that as I'm not in his conference or region. I don't know who you are, or your son, and I'm not even going to assign judgment to your or his actions.
I don't know if there's anything actually in the UW system rules about dismissing a student from an extracurricular activity with a written notice. I think that's probably specifically for concerns regarding employment.
My opinion is that you're thinking that a system of due process is setup for this, but I doubt that. I worked on my former institution's code of conduct, and some sort of right to appeal was not within the system. There was, for lack of a better term, levels of punishment to make sure that punishments between sports were equal. Missing a single basketball game versus a football game, representing a percentage of the competitive season... But the top level punishment was always dismissal.
If you genuinely believe that your son has explicit due process rights in this case, contact a lawyer.
Your point seems to be that your son was unfairly punished for hosting a party. In 1997, may not have been a big deal, but in fall of 2020, this carried a little bit more weight. It's a false equivalency to talk about the people attending getting no punishment, but the person hosting getting all of it.
All of that said, as a coach, you need to have control over your roster. Complete and final. Frankly, I probably would have dismissed your son at the moment the incident came to my attention, but I can understand why that was not done, procedurally; and this delay doesn't necessarily change the trajectory of this outcome.
I was lucky enough to only dismiss two or three athletes from my team. A couple were easily with cause, but one wasn't. One kid didn't do any one specific thing, but rather presented a pattern of behavior that were incompatible with being part of my team. This made it very difficult for them to understand, but there was absolutely no way I was going to die a death of a thousand cuts by one athlete, and one athlete can absolutely poison a team.
And so, my overall question is what would be the best outcome for your son at this point? To be part of a program that the coach doesn't want him to be part of? I understand you're annoyed that he's enrolled and can't participate, but it's no guarantee that he'd be welcomed at another program, and I don't agree that a June transfer timeline is that disadvantageous.
A lot of this, to me, is sour grapes. I think you probably thought there'd be a lot of support for your actions and your son, but it's clear that's not the consensus. And ultimately, I believe the coach needs to be able to have total control over the roster. It was a bad idea for Coach Stanley to give your son false hope of being back on the team, but I'm not sure it's totally clear that he had the ability to make that final decision.
I don't think your position on things is wholly unreasonable, but being part of extracurricular activities is a privilege, not a right. If your position is that someone hosting a party during a pandemic is worthy of dismissal, everything after that is moot.