All one can really do for loved ones mourning a loss is be a ear to listen to and a shoulder to cry on.
That said, I'm sorry for your family's loss.
Many people feel like Death is truly like a reaper, coming to end the lives of their loved ones.
I see it as something else entirely. I look upon death as an entity that shows up to guide the soul to a place where it can find rest, respite, and peace until it must make another journey (either to a higher plane of existence or reincarnation).
Having said that, all I can say now is remember the times you spent with your grandfather and keep his memory alive in your heart.