My dad hits my dog- how can i get him to stop?

My dad physically hits my dog. My german shepherd, Malcolm, is 12 now, he has hip issues, is going somewhat deaf, and is not always sure what is going on. Sometimes he cant help it, and he cant tell when he needs to go out, as you can imagine this means he poops in the house. My dad, with the temper issues, then start yelling and most of the time strike the top of his head or i have even seen him wack the poor dogs hips into the conner of a wall! I tell him he is abusing my best friend but he laughs and says that he is not. I am sick of seeing the dog I grew up with abused. Any suggestions?