Thank you for trying to be the best step mom you can be. I understand that you're overwhelmed, and your stepson's behavior seems to be more than one person can handle.  If you are being given the role of primary care taker- than I encourage you to get some outside help.
-Talk with his school counselor and get their evaluation of your stepson's issues and needs
-Have your stepson diagnosed by a psychiatrist
Take these professional's advice.  It may mean your stepson needs some special provisions at school.  Medication may also help.  It would be great if your husband can attend these appointments with you.  

For you- find some outside help too.  Find a support group of women you can spend time with to rejuvenate your spirits.  It may be your church or synagogue, a club, or even a group at the local gym.  

Hire help so you can get out of the house.  If you husband is able to be "gone quite often" encourage him to share the load, and you can take turns getting out of the house.

I agree with the other answer that your household needs a set of rules, which would apply to all the kids in the house.  (Good idea.) But it sounds as if your stepson needs more than this.

You don't have to do this all yourself. Involve your husband, and seek outside help.
Good luck to you- and thank you again, for wanting what's best for you stepson, and yet realizing that you don't have to sacrifice your sanity to be a great stepmom.

Kindest Regards,
Shirley

Shirley Cress Dudley, MA LPC
Founder of The Blended and Step Family Resource Center