OMG! This makes me cry. I am not really sure what to say, you may have surely destroyed this dog.
Why would Zeus want to listen to you or come when called when there is the possibility of him being whacked when he does come. Spanking is not discipline, it is corporal punishment. Discipline is training and practice to produce specific patterns of behavior. You have not trained Zeus to know the behaviour you expect. Dogs are like children, they need to be taught the expected behaviour to be successful. Zeus is cowering, so yes he fears you.
We rescued our Greta when she was 14 m.o. and she was most likely beaten by her previous owner. We spent many hours and days desensitizing her. It took her a while to trust people and to see her truly smile and to see her rump wiggle with joy. She is now 5 years old and is still occasionally hand shy. Story has it that "she would not listen to the previous owner" she would not come when called. Greta has been scarred for life by her previous owner.
The only advice I can offer is to not raise your hand to him anymore. This is not the way to train him and is setting a poor example for your son, and go to obedience training. I don't spank a living creatures-animal or human, so have not had the need to rehabilate as your situation presents. Maybe someone else here has some insight.