Hi spannerpan, I am so sorry to hear about your grandfather's health. He must be very special to you and it must be hard for you to see him like this.
Your questions unfortunately can only be answered by a doctor who knows your grandfather's life history and all about his disease.
With medication and care he may live long, depensing on the severity of the illness. The question about pain can also be weel answered by a Dr. Who will give you correct and reliable info.
Right now maybe you can help by being strong for your grandfather, take care of him and spending time with him and encouraging him through the hard times.
Hope he feels better and the very best to him.