I'm confused on what the plot/goals of this are. Is this father in love with his son, and is upset that he was NTR'd when he got a girlfriend, and is using the girlfriend's body to mindbreak the son so he can't live without his technique? That even if he stops possessing her, she'll never satisfy him in bed, and only daddy can?
So the father called him "his idiot son" because he was shy so the father possessed the girlfriend to become more slutty and seduce his son so he would crave more and more sex. The problem is that he should pose as the girlfriend in a permanent way (and he enjoys it).