I was the careprovider for my sister who recently died from cancer. Make sure that you take care of yourself. Research the process of dying from cancer so that you are more prepared for the things that your grandfather will go through. Ask medical professionals questions. Hospice has wonderful resources.
Be truthful, but kind; present but not hovering. Keep him as free of pain and as comfortable as possible, with the help of Hospice if possible. Bathe him, soothe his needs. And love him so to be sad but without regrets.
You are extra caring during this time. You contact your local hospice they will help. You keep them as comfortable as possible you tell them all that you can while they are still alive. You keep it as stress free as you can for you and them during this difficulty.