"Why would you ruin perfectly good peanuts by adding candy corn? That's like saying hey, I have these awesome nachos, guess I better add some dryer lint." - Nonny
You don't have to play along with not knowing that things are strained between father and son. This is a difficult time for everyone, including you. You can certainly tell your grandfather that you appreciate his concern but all questions should routed back to your dad. And if he chooses not to discuss it with gramps, you respect that.