Our parish secretary had this battle. She and her family opted for prolonging that battle for as long as possible. When she'd had enough, she announced that she'd had enough, and made her family go along with that.
I knew two Mudcatters well whose cancer was not in the brain, but who faced these decisions. They followed the same path as our parish secretary. A time came when they needed to know it was OK to let go.
I knew one dear old soul whose terminal cancer diagnosis came at the time her early Alzheimers was going into high gear. Her husband had always been a real role model for having cared for her like a champ and a gentleman their whole married life. He chose the path of kindly letting her slip away before her mind was so gone she could not say her goodbyes.
I knew another dear lady whose body had been ravaged by malpractice and surgeries to try to correct the butchering she'd had.... a day came when the docs said they could do no more. She arranged a hospital-based farewell. Lucid as long as possible and then the relief of drugs. A farewell moving from people she'd known all her life coming to visit, memories shared, love, laughter nad tears-- to a brief death watch where her loved ones grew from comforting her to comforting one another.
I've known tough old farmers who have gotten the diagnosis (terminal) in what they thought was the peak of their country/manly strength, and who chose to face it head on, bite the bullet, and let themselves go sooner rather than later. "If I was a cow, I'd put me down," was their attitude, "so why make myself suffer longer than necessary?"
The true but hard thing is, there is no right answer; there is only that which you can bring yourself to do, in doing the best for him that you can.
There also is no way to ensure agreement among all parties, now or later. There is bound to be second-guessing, both sooner and later. Try not to take it personally.
The main thing is to not try to know ahead of time how you or others will or should feel about any of it... just walk through it the best you can, letting it unfold, trying to make decisions from the heart.