As a mom of a son who goes to his father's house every other weekend, I believe your step-son's mom is doing the right thing. She needs to include the father in all of this. There is obviosly something going on and they need to get to the bottom of this. I suggest either sending the boy or the whole family to a therapist if they can't figure out whats wrong. Maybe the change of going back and forth so often isn't doing him any good. Maybe he misses his dad more than everyone realizes. He's acting out for a reason. Obviosly he respects you and his father more than his mother which is wrong (maybe he's learned this from someone?). She needs to start playing the bad guy, but you and your husband do, as well. If he acts up in school, and he get punished by his mom, the punishment should also travel into your house. He has to spend the week in his room with no friends or tv? the same goes for at your house. No one can bend the rules even just a little. You do and he'll fall apart.
I've had to bring my son to therapy before. He would come home from his father's and he would hit me, etc. No reason for it either. Once we started therapy (all of us-me, my son, his father and my husband) he got much better. Even after just 2 sessions. Everyone has to be on board with this. Not just the mom. She would feel like you guys just don't care about the boy at all, just when he's with you and not all the time, that's not fair to anyone.
It can be hard, but there are people out there that can help. You should be happy that she's comfortable enough to call your house and discuss your stepson with you and your husband instead of shutting you out.
I wish you the best of luck. I've sort of been there in a way.