My 10year old stepson only hits me when I’m pregnant!!

Okay so here is the backstory. My stepson was the baby for about 8years and then I had Noah. Now, I’ve been raising all three of my SO’s children for the last 7years full time so they are my children as well. When I was first pregnant with Noah, Peyton punched me in the stomach when I was 15weeks pregnant and my SO did nothing about it. I’m now 11weeks pregnant with the second baby and he hit me again the other day and all my SO did was send him to his room. No talking, no punishing, no nothing, which to me says, “hey it’s okay to hit your stepmother”. I have talked and talked and talked until I’m blue in the face and honestly I don’t know what else to do. I realize Peyton has some issues because he has abused Noah before but we’ve gotten him counseling, I’ve talked until I cant talk anymore, and now I have no idea what else to do. I have to think of the long run and what will it be like when he’s 16, bigger than me, and still hitting on me. Any advice?