Pregnant with first child and feel like marriage is falling apart?

Last year, I got married to my best friend. We had little arguments here and there but who doesn't. I am now 23 weeks pregnant and I feel like my whole marriage is crumbling. He has a son by another woman and if I try to discipline him or get on to him in any way, my husband will discredit me in front of his son. So, now, I am in a position where if I ask for my stepson to do anything or he does something out of line he won't listen to me. Plus, my husband is acting very distant. He used to help me around the house and now he says that is my job because I don't have a physically demanding job as what he does. All he does is stay on the internet playing games on Facebook or playing his Madden game. He doesn't want to go out with me or even talk. Sometimes he will say stuff to hurt my feelings and later act like nothing happened. I don't know if it is my pregnancy hormones causing any of this or maybe everything is crumbling around me. Just need some advice because all I can do right now is cry. I love my husband more than anything in this world. Is there something I can do to help rebuild this relationship?