How do you get a dad to recognize his 4 year old son is rude, disrespectful, and a habitual liar?

My fiance has a son from a previous marriage. His son is 4 years old. I take care of his son because he is a trucker and his sons mom is in Virginia. His dad comes home every other night. His dad and I live together with our newborn son, and my daughter from a previous relationship. My daughter is very respectful towards people and their property. He is not. He is a problem child, that wets the bed, curses, lies, is disrespectful towards me. I have mentioned his behavior to his father, but he constantly is telling him that he is going to handle him, but he never does. I love my fiance, he is the father of my son, but I am ready to call it quits and just be parents because of his previous son. My daughter (6) is beginning to act out in a manner she never has because she sees his son basically getting away with murder. He misbehaves at school, home and when we go to public places. His mom rarely calls him, sees him on holidays, and sends no money but tries to parent.