Hi, I'd like to help you with your question.
It is very hard to deal with a narcissist. Narcissism is a personality disorder which is a category of disorders that are difficult to treat with therapy, even if the person wants help. Everything about a narcissist is "me first". They lack empathy and only want to be involved in something that benefits them in some way. They do not hesitate to say or do hurtful things to someone else. When told about their behavior, they often find ways to make it the other persons fault or they just ignore the information.
There are two ways you can address your stepson. One, you can lower your expectations. Unless he is acting in a harmful way while in your home, you can allow him in, but do not expect him to behave as other guests or family members might. Keep in mind that correcting his behavior or even just asking him to act in a certain way will cause him to react in a hostile way. He does not realize anything he is doing is bothering other people. You can react by praising him in such a way that you do get the response you want. For example, if you want him to do a certain task, tell him that everyone admires the way he does it. Appealing to a narcissists ego usually works.
Try to keep your emotions out of your dealings with him. Narcissists often provoke emotional reacts in others because their behavior is so offensive and hurtful. Training yourself to not react and to remain neutral can help. Develop a few non emotional responses you can provide to just about anything he says to you, especially when he tries to provoke you. "I'm sorry you feel that way" is a good response to just about any comment. It leaves your feelings out of it, negates a reaction and keeps him from feeling offended.
If you feel you still do not want to deal with him at all, then it is alright for you to ask that he not be in your home, especially if he is acting out. Or you can be sure you are not there when he comes to visit his father. You can also ask that your husband see him outside the home. This keeps him from your home but allows your husband to continue contact and maintain the relationship.
You can also learn more about narcissism to help you find more ways to cope with your stepson. Here are some resources that will help:
The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship in Work, Love, and Family by Eleanor D. Payson
Disarming the Narcissist: Surviving and Thriving with the Self-Absorbed by Wendy T. Behary
The Object of My Affection Is in My Reflection: Coping with Narcissists by Rokelle Lerner
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped you,