D.M.
What you can do depends on your relationship with your son and your hubby's expectations of you. If he is getting defensive with your suggestions alone then he is not going to back you up if you step in. When my step-son had these issues WE told him that he needed to shape up or we would be homeschooling. We had to back that up but I am confident he is getting a good education and he is now a very responsible boy. I've also been in his life since he was 3 tho so he has long since accepted my ability to parent him.
You want to let you husband know that you will support his desisions with you step-son no matter what but he does not have to do it alone. You two are a TEAM! He does not have to make all the decisions, or be the only parent. He just needs you on the same page. Ask what he expects from you and how much authority you have. If your teen does something wrong while he is at work can you ground him? send him to his room? yell? If you do these things will your hubby come home and undo it or support you?
My boy and I have alot of fun with some of this. At 6'3" there is not much I could do to stop him but comonly if he gets smart or sassy I'll say "you'd better knock it off. I'll climb a chair and knock you upside the head if I have to!" He'll usualy laugh and this will stop what ever I saw comming. Sometimes he'll run off and pout though.
One thing to realize with all this. He has stood up to his mom and her boyfriend. He has no respect for either and if they tried something like that things would get physical. He accepts my words because he dad had fostered our relationship and backed me up always even telling him that he had to give me at least the respect of a babysitter because he has put me in charge.
Once you get your hubbies back up go to fosterparents.com and order some courses for anything that sounds like him. They are cheap and will give you the prospective of someone who does not have a realtionship built up.
I hope this will help! Step parenting is hard especialy when your the only caring mom available!