I couldn't find your last request to find out how much of the time you have your step-son. If you're the primary care takers and he just visits his mom then what you do will eventually be the most important influence. There really is nothing you can do about what the mother does as I'm sure you've found out.
I've adopted a foster child whom I've reised since she was 7. She did those sort of things to me, I'm told because I was the safest person for her to do them to. She knew I wouldn't get rid of her. And she was so very angry because of having been abused and passed around. Your step son is not doubt also very angry. Divorces do that to kids and when they do not know healthy ways to express that angry they hurt either themselves or other people. I was told that in a way it was better that my daughter hurt other people instead of herself. If she was being nice on the outside and thinking of hurting herself we might not have been aware that we needed to help her. Some consolation, huh. :(
What does the counselor recommend? He's had training and experience with this.
From my experience and what I've learned from the counselors is that punishment does not work. It only increases the anger. Washing out his mouth with soap does not tell him why he should not lie. It does tell him that lying makes you angry and increases his anger. I've learned that if consequences which teach are much more helpful.
And thus consequences that are related to the wrong action are the better way. And consequences that are delivered in a calm and even loving way are even better.
I used a parenting book entitled Parent Effectiveness Training which helped me a lot. There is more to the title and I can't tell you the author's name either because my daughter has it now. She's 26, has 2 children, and has successfully learned to manage her anger and her actions. So there is hope.
This book and others give examples of logical consequences. If he slaps or spits at school they are the one to give him consequences. You support the school's decision. At home, it is reasonable to take away one privilege or activity and arrange for him to have a way to get it back. You could take away one more for each misbehavior after that. But to take away all of them to create a week without any fun is counter productive. How would you feel and react if that happened to you? Angry, I bet. And it's anger that you're trying to help him get control of.
I agree that taking him out to dinner and for ice cream is not the right thing to do. But I do recommend that you spend more time with him having fun when he is behaving. In addition to being angry I suspect he is also trying to get more attention. Being 1 in 5 and being the outsider with you is very difficult.
All problems within a family are the result of family dynamics. The child with the obvious problem is called the scapegoat. And so I'd recommend some family counseling. Start out with just you and your husband and see what the counselor suggests. Or if he won't go just you go and get some information.
My daughter hit me because I allowed her to do so. I was told that over and over and I kept trying different things but none of them worked for years. I finally realized that even tho I gave consequences and thought I was giving her the message that hitting me was not alright I was causing some of her behavior by responding to her words in a defensive manner. I wasn't just listening to her when listening was all that she needed. I was focused on discipline. Once I learned to not respond to her angry remarks, such as you're not my mother and can't tell me what to do, by ignoring what she said she had no reason to hit me. I still stuck to the rule quietly inside my head at that moment but didn't try to explain anything about it. I could then provide a consequence when she did break the rule in a calm, unemotional and non-argumentative way. It took me years to learn that. I kept wanting to convince her to agree with me and to do what I said because she could understand it and agree. We had lots of arguments. Once I stopped arguing she stopped hitting me. Now I can see how frustrating it is to have someone tell a child that they are right and the child is wrong. Who is right doesn't matter. What is important is that the child obeys.
I don't know if that helps or even fits what is going on with you. It's just what worked for me. And I'm suggesting that you can use that idea for his actions other than hitting since he doesn't hit you.
Another thing that I learned is that if I include the child with writing down the rules and decide on the consequences as well as have only a few rules, that the child has an incentive to follow the rules. Of course one of the rules is to never hit or hurt someone. These rules are different from chores although you can set up chores in the same way.
And positive attention is much more effective than negative attention. Catch him doing good as much as you can and praise him. When children do not get enough (and they are the one who decides what is enough) positive attention they will do something to get negative attention. And I'm guessing that because he is a child involved in a divorce he needs a whole lot more attention than your children who have been with you always even if yours also went thru a divorce. In addition to the anger he feels insecure. Punishment will make him feel even more unloved. Discipline will help him learn. There may not seem to be a difference between the two but there is. Discipline is done calmly and lovingly because there is something the child needs to learn. Punishment is frequently, tho not always, done in frustration and/or anger and is done to show the child he has done something wrong.
I know that remaining calm and feeling loving is extremely difficult. That is why sometimes we need to just send them to their room until we can be calm ourselves and then deal with the discipline. Sometimes just sending them to their room to think about what they've done while we think about it too (letting them know that is what we're doing sometimes) and then discussing what has happened is an effective disciplinary lesson all it's own.
I wish you luck. You are in a very complicated and emotional relationship with your stepson. Even if her weren't so angry being a step-mother is most difficult. I've been a step-mother too.
And I wasn't successful at it because my husband, the father, and I weren't on the same page. We couldn't agree on how to handle things and agreement in front of the child is essential.