Son told grandpa "Why haven't you caught corona yet"

So I dont know if anyone remembers but a few months back I posted that my 17 y/o adopted black son and my father constantly clash. My father is old fashioned and still unhappy that I brought in a something from a different race so he will usually verbalize those feelings towards my son. Today my father made a joke to him about lynching being legal back in his day. I told my dad off and called him distasteful. I was hoping that my son would ignore it because my dad is just trying to piss him off but he goes "Why haven't you caught coronavirus yet?" To a 70 year old man. Whose age group has high death rates from the virus. I slapped my son (Not hard) and he raised his hand as if he was gonna hit mw back but put it back down. I sent him to his room because I was shocked. Once he was gone I told my dad that he needs to be a better person or else he's gonna have a lonely funeral when that time comes. He just told me not to talk to him that way because he's still my parent no matter how old I get. I just don't know what to do to make these two get along.