Problems with my stepson. ?

I've. Been married for two years, and my husband has a five year old son. Before we got married I met him and noticed right away that he was very disrespectful and had no manners. I wanted to help my husband raise him because he trys to be his friend, but that boy needs a father since his mom is never there for him. However, everytime I try to correct the boy or let him know he's done something wrong, my husband bursts out at me as if I've done something wrong. I don't think its fair because he brings him to our house, and Im not allowed to say anything if he's misbehaving. I suppose ive develoled some sort of anger twords this, and its gotten to the point that I dont want him here because when he is, I turn into the enemy in my husbands eyes. I wanna get along with the boy, for my husbands sake, but even though its his dads fault, I can accept him. What can I do to better get long with him? I'm desprate because hes here today and I want it to be happy new year!