Jackson, take a notebook and pen with you when you go see him. Ask him to talk about his life-everything, his regrets, feelings, experiences, people he has known, all the places he has lived, where he met your mom- everything you can think of to ask him. All the places he worked. What he did as a kid. You get the idea. Write as much and as fast as you can. Even better would be one of those digital voice recorders, even if he fills up two or three of them. Having him talk will give a sense of closure to you both, as he and you share his life one last time. The time will come when your kids want to know about grandpa, and you will be able to tell them.
And above all else. let him know of Jesus' death for him- that his sins are forgiven with a simple prayer of trust in what Christ did. If he has never been a religious man, read the story of the thief on the cross to him. Go for it, dude.