Hello AParent - and welcome to A2K
I would rarely respond to posts about children as I am in no position to give a non-bias approach based on my own personal experiences.
However, I feel I would like to say a couple of things.
Your son, only by what you have stated, sounds rather like a petulant child who has Mom and Step-Dad living by his rules... and this, as you're finding cannot go on. Instead of stamping his feet - he appears to be using his education to get you to submit to his superior intellect. Of course, it could be a BLUFF - just like younger teenagers do... but not on such serious issues.
Firstly, I won't/can't/choose not to agree re the mental health disorders as I think this is a very tricky subject and may not be the best way forward into resolving this situation. It just puts a lot of ideas into your head which may not be true.
However, he appears to be acting like a child - so, I would treat him like a child. If he is disrespectful etc - start making the rules and enforce them. I don't see why you are paying for his clothing and transporting him around. Just say NO. If he is disrespecful - well, tell him to get to wherever he's going under his own steam - you do not have to be his taxi service, you do not have to provide his clothing - he's 21!!!!!!! If of course, he was living at home, pulling his weight and being a decent and responsible member of the family, well, obviously circumstances would be different.
Also, again, from what you state, it sounds to me that he is lauding his "criminal degree" over you. I think he is possibly even playing you for fools (seriously, I do not mean that in a disrespectful way) - of course, he insists he knows all the law and his rights and constitution (fer goodness sake)... and he does not expect you "not to believe" him. He does sound very clever.
We do have some experts here in law and I am hoping they will see your thread and give you correct legal information if possible. I would advise you to seek your own counsel asap for clarification of the issues and please don't start saying things about mental health issues etc as this can lead to opening a rather large can of worms. If you have personal friends who are lawyers, who also know your child, talk to them... and if necessary retain some professional advice.
He sound to me as he thinks he rules the roost. He doesn't. You, as his parents do. Live by your rules... or he moves out and get a place to live by his rules. If he has no money - he best go get a job to earn some dosh before going back to college.
Be strong, firm and assertive. Please don't allow him to bully you by using the education you have provided him the opportunity of having, forcing you to sumbit to his wishes. Talk to counsel who can be specific with you about specific issues.
As I say, we have a couple law experts here, so hopefully they will see your post and will be able to give you an idea of how to proceed now.
I do wish you well. Being a parent is so very hard at times - being a responsible parent who teaches their child what is acceptable, and what is not acceptable, even at 21 living in the home, well, he needs to start maturing just a tad. It will be difficult for you - don't let him bully you by using clever words.