Not pregnancy related but I could use advice.

Me & my husband got married March of this year. I'm 35 weeks pregnant and I have a 2 1/2 year old. My husband is awful to my son, he's not his biologically. But he's been in his life for a while. He literally snaps on my son for NO reason, he constantly yells, he spanks my son too hard and when I step in he yells at me and degrades me. He even told me if I'd "bust his butt" he wouldn't have to. My husband's parents were never parents to him, they're pill heads.. I'm so scared to bring a newborn into the world and him get worse to my son or take his anger out on me & baby.. idk what to do.. If I try to talk to him, it always ends in an argument.. idk what to say, or what to do. Me & my son love this man, but I can't deal with this.