My son is 13 (stb 14) and until recently has lived 50% of the time with his father and 50% of his time with me?

Okay, well I grew up with seperated parents and what you have to understand is it's really hard for children with separated parents. When i was a kid my brother and I hated our step mom, because she would do stupid things and got really jealous of the relationship my father and i had (i was a daddy's girl and like a carbon copy of him), but even if she didnt, i still wouldnt have liked her, because she wasn't my mom and she was in my moms space. He is probably wishing that you and his dad would get back together, and with step dad 'in the way' that isnt going to happen. Obviously you have no intention of it happening step dad or no step dad, but he won't get this and even if he does, it won't change his opinion. It might not even be this, he might just want you all to himself, and feel as if step dad is taking you away for him. One thing for sure, is you need to talk to his dad and make sure you both enforce punishments and consequences equally, so you're not ending up with the big bad wolf title. You might want dad to have some father son time and talk to him about respect, and dealing with anger, how to tell people when you're upset and explain what's wrong rather than shoving/hitting them. But don't baby or patronize him! Also bear in mind that he will have a lot of pride and you cannot hurt it, so breach the subject tactfully. You need to stop treating him like a child and more like a teenager or young adult, i know for sure that my little brother was mommy's baby his whole life and my mom always treated him like he was a baby, and my sister and i like we were older teenagers or adults from about age 10 in terms of responsibilities and behaviour. You need to explain to him the situation, how you aren't going to get back with his dad (make sure you dont diss his dad because right now he probably idolizes him and this will just make him not trust you) and make sure that you can spend time with him just you two, or just you two and his sister, so that he knows that you are still there for him even if he isnt comfortable with step dad. Maybe you should ask dad if you can spend a day with you, him, and the kids? If your son can see his father (someone he looks up to and respects) respecting you maybe he will start to respect you! I'm sure he will get over this stage but you have to deal with this very carefully, and don't be harsh, be firm and always make sure he knows you love him and he can come to you with anything and you wont get mad, you will help him. Good luck!