Thats a tough choice, but as you already stated keep him comfortable and hydrated. Pain meds are terrible, they make you constipated and mentally out of touch. When he is ready to go I'm sure he will let you know.
I watched my mother in law die of cancer and it was hard but that is what she wanted, to die at home with family. Tha last two days were the worst, pain meds and keeping her hydrated and when her time was up God took her home. As Christians believe like your grandpa that there is more than this life here on earth, an eternity with God. Until then we wait it out until its time to go home.