Prayers for you both. I know first hand how difficult and painful the final days are. I cared for my grandmother as she was under hospice care at home for the last two months of her life. You and yours are fortunate in that you have the opportunity to say anything your heart needs to say before he passes. Take the chance to have the final moments your heart will need for closure later. Praying for his peaceful and painless passing.
Hopefully. You still have a grandpa, luckily. Hell, do you still have a dad? I mean, my life has been one living hell. I've had my Dad's Dad, my Dad, my Dad's Mom and my dog all die this decade. Shit is rough.