I am really sorry you are in this situation op, particularly as you are pregnant as well. It must be very daunting to be faced with this now, if things have been okay up to this point.
I was raised by a father that hated me - he was my real father, and he did exactly what your dp is doing to your ds (and worse). I am just having therapy now, I have never and will never get over it. It shattered my confidence, self esteem, my childhood and the first part of adulthood. It has taken me decades to even manage to live with it. I am extremely upset my mother allowed it to happen to me. I do hold her at least partially responsible.
This can not go on op.
Your son will get older, and get angry and you will end up with full on physical fights and worse. Your son will soon become a teenager and become even more challenging than he is now. You have to protect him, whatever it personally costs you. Your other children will grow up in the shadow of all the stress, violence and anger it will ruin their childhood too. There is no chance of a happy ending with this, zero.
Please leave before it is too late. Your ds may never forget what your dp is doing, but he will know you had the courage to stand up for him, to protect him and to take him out of the hostile environment that should be his home. You have to do this for your kids op.