Given all of the family structure changes your step kids have and are attempting to adjust to, it is my guess that your stepson's acting out behavior may have something to do with adjustment issues. It would be my strong recommendation that you seek mental health individual and family counseling. This would not only help your stepkids cope with the number of adjustments in their life but also help you better manage them. The school counselor might be able to provide these services.
Sometimes, parents take it for granted that their kids will simply adjust to multiple parental authorities and drastic family structure changes. However, this rarely is the case. Discipline needs to be in place. However, these kids require emotional support systems to emotionally adjust to all of the changes in their life. Without it, discipline isn't going to do much good.
Thank you for your advice, I have made an appointment wit the school counselor to talk about the issue.
Sounds like to me he didn't have a stable mother so he's looking at you like you will hurt him the same way so he has issues with women