Refusing to do anything

We are new to TSG, I’ve read the book Roles, Popular Parenting Methods and I’ve started Parenting A House United. I’ve also watched the first module of Teaching Self Government Video Course.
Our family is a huge mess, my husband and I are not on the same page with anything. He is borderline verbally abusive to the children. Name calling, shaming etc. He is an ex drug addict (10.5 years clean) as well as dealing with several medical conditions he refuses to properly treat. I’m not telling all this to make excuses just to give background information on the situation. I’ve recently made some major shifts and realized I wasn’t being the mom I should have been. I have been a poor parent for the last 14 years and now see that. I’m working on making changes to change myself for my kids.
We have 12 children ages 23-1. 10 still live at home ages 19-1. My question today is about our 9th child, he is 7, almost 8. He is suspected ADHD and ODD. We are trying to get him in for an evaluation and exact DX.
So I’m totally lost on how to try to teach him the TSG way. He just refuses to do anything. Tonight he refused to shower. It’s been 4 days since I forced him to shower last. I tried the whole I’m going to give you an instruction now and you are going to say Yes Mom and go do it. Okay? He started before I even gave the instruction he was saying no, you can’t make me do anything. I don’t have to listen to you. Finally I said you are going to take a shower or a bath your choice. He said no. I said if you don’t pick I will have to pick for you. He said no. I said ok shower it is. I took him to the bathroom and told him he could undress himself or I would help him. He refused and said no. I wrestled off his shirt while he tried to punch, bite and kick me. I got his shirt off and gave him the option to finish undressing himself get into the shower himself or I would help him. Well he still refused so I picked him up half dressed and put him into the shower. It was a battle him calling me names screaming he hates me etc but eventually he was washed.
I can’t give him chores or even most consequences because he won’t do them. He won’t take breaks (what we call the calm down spot) he is physically violent towards me when I try to enforce an instruction. I’m lost and at the end of my rope. I’m not good at staying calm yet but I can’t just walk away from a lot of these issues, sometimes it’s him physically hurting siblings. Where can I start to make this work. Help!!