Struggling with being a step mom & now a new mom. Advice?

Can any moms in similar situations give me some advice. I recently found out I'm going to be having a baby for the first time. My husband already has a child from a previous relationship, I've been in this child's life for a little over 2 years now. Our relationship in the beginning was great, only recently has his son started to resent me. He disrespects me all the time, gives me attitude, tells me he doesnt love me, and I'm not his mother. Which I understand, I'm not his biological mother nor would I ever try to replace her. I'm trying to resolve our issues because here in the next month we are taking over primary custody of this child. On top of that I'm pregnant right now, and I dont want him to lash out on me or the new baby. Any advice would be much appreciated