I'm not a step mom but my husband is a stepson. Everyone I know from his family says he was a very good respectful child, even his step mom. However, he does not like his step mom. He does not like her position in the family. And that is a reaction from a "good kid," so imagine the honest reaction from a not so "good kid." You should keep in mind that no matter, how you react or treat him he is never going to see you as his mom. That is probably a huge part of the problem. He does not want to let you in, does not want you to try to play the part of mom, does not want to give you the idea that he is letting you play that part. It is also not an easy thing for a kid to know how to react to the tough situation of divorce, a new woman for his dad, marriage to a woman that is not his mom, and the whole ball of wax. I'm just imagining how I would have felt in that situation, when I was a kid. Discipline in his eyes is most likely his mom or dad's position, so he doesn't want you taking that position. That would mean that you are playing the part of his mom, which he doesn't want. That is the catch 22 that makes it almost impossible to win.
I do not think it is impossible to find an arrangement that can work for everyone, even if you and dad have different parenting styles, even if every other step mom says it's not going to work. You have to communicate, especially with your husband and him with you. Have a talk with him while he is not busy or engrossed in anything. Even if you have to schedule it in with him. Have a few talks with him. Tell him you do not want to take his or the boy's mom's place, but you have to be able to guide him and discipline him when you are around. Explain that whatever happens the boy needs to know what is expected of him, and what the consequences are for not following those expectations. Remind him that even teachers have to provide discipline sometimes and you sometimes do too. If he feels you are too hard on the boy, explain that he still needs discipline and that both you and your husband need to find rules and consequences that you both can compromise on. You may have to give in a bit, but that's what compromise is about. If both of you have a plan, and have set regulations and consequences for your stepson and any other children to follow, then both of you can explain together that these are the rules, these are consequences for breaking those rules. Even if you have to allow dad to take care of some of it, make it a set of rules that need to be followed. Discipline doesn't have to be punishing, it can be as simple as trying to find out what the real problem is and looking for a solution that will help with that problem. It's probably a good idea to allow the boy some say in the rules, and consequences, or at least allow him the chance to explain himself and his problems or feelings on whatever issue it is.
Another way to tackle problems is to agree that when the boy does something to get into trouble or you aren't sure about, you wait until his dad is home and you and dad discuss the matter together and try to come up with a way to deal with it together as a team. As soon as the boy does something that you feel you need to discuss with his father, just tell him, "Ok, I will discuss this with your father when he gets home."
Even being the mom, I find it easier and more comfortable to discuss discipline problems with my husband and try to come up with solutions or actions together. I also think it's important to ask the child for input, not in an offensive manner but in a caring manner. Just kind of matter of fact, what's the matter? is everything ok? why did this happen? what do you think you should do to make this better, or to do better next time? what do you think we should do about this? will that help the problem? will it fix the problem? Then come up with a plan of action, and go for it or have him go for it, then later evaluate how things went, discuss if there may have been a better way to handle it. I think discussions like this make it much easier for the child not to feel angry, or put on the spot, also make it more likely he will try to work on the problems himself, or come to one of you when he has a problem that he can't resolve.
Something that I have been doing with my family for a few months now, is to have meetings. We try to have them weekly, but they probably actually happen every two weeks or so. It has helped though. It gives us all a chance to each say what went well sense the last meeting, what did not, offer ideas to try, see how those ideas are going. I'm not talking a straight up "air our dirty laundry" session. I usually have a set of questions; we each answer each question, then move on to the next question. If things get too carried away, we usually agree that that's enough for this time, and we can go over more next meeting. This has also helped us to feel like we are more together, more of a team, more on one page, and we all have some say in things.
I wish you the best of luck with everything. I hope you are able to find a better arrangement. And I hope you have a wonderful wedding, and marriage.
V.