My heart goes out to you, Dr. A. I've lost both of my parents, and it really isn't easy.
One thing I've learned is that it's important to give the person a chance to talk about dying. If you consistently pretend everything is fine, your grandfather may not feel able to talk openly with you. It was helpful for me and for my mother and dad to be able to talk about it, and be sure we'd said everything we wanted to say to one another. I cherish those conversations now, and I've never had that feeling of "I wish I'd told Mother this while I could."
Take every chance you can to make memories. Video your GF telling stories about his life or history as he sees it. Then you can share those with your children and grandchildren in the future.
I hope the journey is peaceful for both of you.