Please someone help me

Me and my boyfriend are expecting a little one in August. Well, my boyfriend has a four year old whom we have full custody of. Well, I have tried and tried to be the mother he doesn't have (she was a drug addict and he was born addicted to drugs and had to be in the NICU for a week) well at this point he does everything in his power to make me upset, angry, eat. If his dad comes home he will look me right in the eyes and do exactly what he was told not to. He yells at ME like I'm the child, he hits, everything!! And me being pregnant now, I am having a way harder time dealing with this. My boyfriend just tells me to spank him but I would have to spank him multiple times throughout the day because of how much he disrespects me! I'm just having a hard time, I'm stressed to the max and just want to cry and stay away from him. He's FOUR!!!