4 year old step son won’t poop on the toilet

He pees just fine. No issues there. But when it comes to pooping, he absolutely won’t do it. He screams and cries, tells us no, throws himself around. We tried the reward system, the taking things away system. We sit with him in there. We tried books and games. His father got a butt dial one evening when the son was at his moms. She was screaming at the top of her lungs at him trying to get him to poop. I think she messed this whole process up for her son. Ever since, we have always tried to be calm and patient. Nothing is working. Any mommas had this? What can we do? I suggested therapy and seeing a doctor but his father doesn’t quite agree yet. I’m pregnant now and would really like him to stop using diapers before May, when this baby is born. I’m open to any and all suggestions. This household has so much tension because of this. We all end up crying and going to bed exhausted. In the end, my husband caves and gives him a diaper. He doesn’t want him to get fissures. I told my husband that I can’t be apart of this because it’s stressing me out too much. I’m pregnant and can’t deal with this 4 year old screaming at me and throughout the whole house. I don’t want our baby to be born into a hostile environment. I’m so upset over this. My husband cries because he feels like a failure. My hands are tied.