21 weeks pregnant and being a step mommy...

I'm 21 years old and half way through my first pregnancy after having two miscarriages. I got married last year in August and have been with my husband for 2 years. I have a step son who turns 3 next month. Everything is changing so much and I know it's not all my horomones... I really feel like my husband doesnt feel attracted to me anymore, or even want to be around me for that matter. I say one word to my step son and he gives me a dirty look. He doesn't seem excited about the baby at all and we hardly talk anymore or touch each other. My step sons bio mom brags about how well he does when he's with her, then comes with us and he is very whiney and throws fits about everything possible. He can't go to the bathroom alone then cries and almost goes in his pants. It's so frustrating. My husband asks me why I'm so mean to him. He clearly doesn't understand that it's a hard fucking job! I'm the only one that disciplines him and I really think that it's going to ruin our marriage whether we have a baby on the way or not.... I've been around almost his whole life, so please don't tell me he isn't used to me! And he is beyond excited for the baby, he's been doing this for 6 months or so now.