K.O.
I have dealt with a similiar situation. My stepdaughter was 6 when my husband and I got married and had our first son together. She was extremly jealous of the baby and of me.
You have to understand that children from divorced parents always carry whith them this idea that their parents will get back together. My husband and his ex divorced when my stepdaughter was 2, so she has no memory of what it was like to have both parents together.
My stepdaughter is now 11, and still does not fully understand why her parents can't be together. She has since rebelled against me is often mean to her brothers, but can be the most loving child when she wants to be.
Insights I have learned along the way and am still learning:
1. There is nothing that you can do. You need to leave discipline issues up to your husband. Have your husband talk to his son's mother about the behavior and the best solution is to create a united front and let hime know that although his parents are not together they are still going to parent him together. This is extremly frustrating for me because my stepdaughter lives with us full-time and I am the primary caregiver. But it will save your sanity. for example, I will handle immediate problems, so say she is fighting with one of her brothers ,I will intevene. But if it something that happens that I am told about later or homework issues, etc. Dad takes over.
2. Therapy, counseling. ALL kids from separated parents NEED to be in therapy to cope with the changes that they don't understand or know how to deal with.
3. You stepson is crying out for attention. He feels threatened by the fact that your son lives with his dad while he doesn't and that is why he is acting out. The only thing that you can do is talk to your son, make sure that he is communicating to you any problems he is having with your stepson. As for you, as hard as it may seem when the stepkids are being difficult, disruptive, and causing nothing but problems, you have to give them love. Show them affecton, and that you care.
4.Don't overreact or show emotion to negative behavior, because they will thrive off of it and continue to do it. But say your stepson is really good at something or does good on a project in school, etc. have your husband and you make a really BIG deal about it. I'm talking over the top, letting hime him pick dinner for the night or getting cupcakes to celebrate, etc. But if you give a lot of attention to the negative behavior he will continue to do it
because he knows that is what is getting dad's undivided attention.
I hoep this helps. Best of luck and Happy Mother's Day!