Wow, that sounds like a bad situation! I’m very sorry to hear you are struggling with this. I have to admit that the whole situation is outside my experience, including the poo issue (!), but here are my thoughts from I and family has experienced about ADHD:
– Many people with ADHD experience very low arousal and boredom feels truly agonizing. An adrenaline rush helps this state, so they will pick fights as a form of therapy and they perceive it as not being able to stop themselves. Does he have any hobby or activity that he enjoys doing? It may even be computer games; anything where he could build mastery is ideal. Catching him at the start of an episode and redirecting him to an activity that he enjoys may teach him to self-medicate a different way.
– Again, many people with ADHD sometimes cannot stop doing something that they think is fun despite disapproval, especially if there are no consequences or they feel the consequences are business as usual. The consequences have to be related to the offense — do not cut computer time if they do not behave responsibly in the shower, for example. If they don’t behave responsibly during an activity, there have to be limits and close supervision during that activity. Yes, it may not be fun for all, so pick your battles wisely and address one issue at a time. One battle won will have a lot of positive impact.
– For people with ADHD there is often very little that goes their way, often for reasons they can’t control, and it’s not because they don’t make an effort. A blow up may be the last straw of a set of difficulties. If they keep falling behind and being disapproved of, they may conclude that nothing they can do is good enough so there is no point of trying. Try to catch him doing something right, even if it’s small and should be a given for his age.
– Look into ODD (oppositional defiant disorder). This site has a lot of resources, if you do a search. No ADHD person I’ve met is ever comfortable with authority, and taking a hard line with no explanation based on status will always result in non-cooperation. I presume ODD could be this tendency taken to the limit, and sounds like what you are describing. I do not have personal experience though.