What do you do when your grandfather is suffering from cancer?

Be supportive. Having cancer is scary. One of the issues most people have is being afraid of facing the disease and treatment alone. Let your grandfather know you are there and give him lots of hugs and kisses. Knowing he has people in his corner will make traveling down his path easier.
Support. If you really want to help, give all moral support he needs, if necessary financial support, and provide best possible treatment for his care.