Hello Jim:
First of all, you are clearly a loving son and your Dad is very fortunate. In addition to soothing his own fears, the care and love you are giving him will comfort you later. I believe that is the 'gift' that older parents give their children...the opportunity for the children to in some way return the love and care provided growing up and through the years. You will never regret any time spent caring for him now.
Both of my parents died of cancer: Mom from bladder cancer (undetected--or hidden--for many, many years...certainly a far cry from with the wonderful people on this forum) and my Dad of prostate cancer. I have no regrets with their decisions to end or bypass treatment. Yes, I would love to have them here, but the sickness, the weakness, the fear during quiet lonely nights...enough was clearly enough. Of course, respecting life is the ideal, but sometimes "life" and "living" are two different things. In both my parents' cases, the opportunity to survive another 3-4 months was not worth the hard work getting through chemo so each went home and passed away surrounded by love. Hospice was wonderful; angels on earth. They carry the heavy load so you can do your job....which is taking care of your Dad on a mutually-emotional level.
Jim, I hope I'm not stepping out of line; you should feel no guilt in whatever the decision is. I don't think either scenario is a "win", but on the other hand, maybe neither is a "lose" either. If the decision is to let nature takes its course, there is plenty of smiles and moments of joy yet to be had. Those moments will stay with you forever. Chemo at this age may deprive both of you of that...just something to consider.
Everyone on this board will support the decision. Please keep us posted.
Debbie