I have commented before but thought I would weigh in one more time.
Medications for depression have come so far in my lifetime and the advent of SSRI’s was life changing for so many people. That being said you have to find the right SSRI for you. Unfortunately it can be a little frustrating because it can take up to six weeks to fully get in your system.
I tried Prozac and it was not a good fit but Zoloft was and in combination with Wellbutrin and Buspar it has been life changing. I am also in my 70’s and I can’t imagine being depressed all the time. He doesn’t have to live this way and it is certainly not fair to you. If he also suffers from anxiety Klonopin/ Clonazepam might help with those symptoms. Yes they are addictive and withdrawal can be tough but living the way he is now is no way to live the remaining years of his life and your life as well.
Finally constant depression can lead to dementia and if he is miserable now life with dementia would be intolerable.
Good luck to you and congratulations on being a cancer survivor. If you have another bedroom move in there and live your life to the fullest. Nobody says you have to around someone who refuses to get help. Maybe tough love is called for here.