Sorry to be reading your Grandad has been recently diagnosed with bowel Cancer. Bowel cancer took my Dads life in August this year due to it spreading to his lungs, liver and brain. I'm only 28 and feel robbed of a future with my Dad. Do you know what stage your Grandads cancer is at? What the treatment plan will be? Or has that yet to come? I can't offer a great deal of advice because there isn't much we can do apart from support the person affected. Your Grandad will need all the help and support he can get as I am sure he is terrified and worried himself. 

Hi there.  I am sorry to hear of your grandad's cancer and how distressed this has made you.  I am guessing from your post that he is the nearest to a father figure you have?  Your distress is entirely normal and of course painful.  Do you live near to your grandad?  Is your grandma still with him?  Do you have other family and friends with whom you can share your sadness?  It occurs to me on the basis of the information in your post that you might find a good way to repay your grandad's kindness to you by making yourself a part of his care system at this time such as seeing him regularly and doing odd bits and pieces to make him more comfortable and happy?  As Butterfly89 has said once you know more then you will learn what he needs and how it will be provided.  My best wishes to you.