If you are 19 and he is 13, he is not going to respect you as an authority figure until he is totally grounded in unconditional love from you. And maybe not even then. I can imagine how exhausted you are. If I were you, I would spend my energy trying to teach him from your own experience in life rather than "you should," and just leave it with him to process on his own. In the mean time, if you can show him love and continually talk to him about the good things you see, what potential you believe he has, in very specific terms, he will begin to seek you out voluntarily.
When he comes to you, then you have an open door to talk with him about your concerns. If you can share "why" you are concerned rather than just tell him "what" you are concerned about, he may respect your opinion. If you draw on your own life experience, even if it means admitting you've made choices that have made life harder than it could have been, he can't argue with it. He can say, "but that's not my life," but you can say, "if you continue down this path, it could be." There is no shame in that!!! It is called learning from our trials.
I know that married life is suddenly not all you thought it would be and you probably feel incredibly disgusted with your husband over this issue, but this is an opportunity for you to think about, inside your heart of hearts, what you expected and how much of that was mature, realistic expectation vs. fairy tale dreams. That doesn't mean that you were foolish! It only means that you have to re-evaluate how you deal with your life from this point forward.
You don't have to quit and walk away, just ask God for guidance. He will give you all the wisdom you need if you walk with him. If God put you in your stepson's life because you are the only one who could love him enough to be firm with him, then God will bring about the respect for you necessary to make that a reality. You just have to seek Him out for the daily steps.