How can you allow your husband to abuse a child with a belt, what kind of mother are you? Where is the protection a mother has for her children. You said "Also, he spanks my oldest with a belt. I'm not against spanking like a little swat on the butt with pants on but nothing too hard but he is all into spanking with a belt. What, does he undress your son before he beat him, if he does, what psychological damage is that going to cause your son?

Plead with your family again, or just send your son to them so he doesn't have to put up with the abuse from a stepfather. Go to a battered women's shelter, find out what services are available for you. File for divorce, apply for child support and alimony immediately.
What messages are you sending to your children, that anger, abuse and fighting is normal behavior?