Sorry to hear about it. It may not help, but here is pretty much how things went down with my grandfather.
My grandfather went from "simple" mild parkinsons to wheelchair ops in a home in about 3 months. He unexpectedly "stabilized" in a pretty degraded state, and lasted about 5 years at the home. Maybe it was because the place was so nice (not kidding). While there he routinely got stubborn and did strange things you'd expect of a 3 or 4 year old. My Mom accidentally mentioned that his hearing aids looked a bit like cashews and a few days later he ate his hearing aid after having been given some mixed nuts during lunch. The world became very strange to him as his mind increasingly distorted his perceptions and thoughts. What finally killed him was he snuck out to try to go to the bathroom by himself, fell while maneuvering from his chair to the toilet, and broke a hip while stuck between the toilet and wall, or maybe from sitting down abruptly after trying to get up out of his chair. They found him on the floor in the bathroom after a room check maybe an hour later. He complained of hip pain but it still took a couple of weeks to diagnose it as a broken hip. He had a violent reaction to the anesthetic used to knock him out to work on the broken hip, and it took almost a month to get him out of the hospital. He never really came back after that but refused to give up and required invalid care but nothing really unusual. He degraded slowly but steadily and had to go back to the hospital due to various issues and to get his medications re-balanced. But the second day in the hospital he refused to eat, went into a coma a day later and died 3 days after that.
When the body is that old and frail, the time from deciding to not eat and death can be just a few days. It is a decision that can't be taken back due to the severe damage that happens to internal organs almost immediately. Make SURE the advanced care directive has something in there about what to do in that situation. If he had directed to keep him alive at all costs, he may have lived in the coma for another few years because he was an intensely stubborn man. What is certain is that his quality of life was already well below what he wanted to preserve and keeping him alive would never result in anything but a horrible barely alive condition. His dementia made the world very scary for him and being trapped in a horror movie while confined to a hospital bed would be nothing but torture. Fortunately we documented his wishes long before it got to that so reducing his care to a simple IV drip after he entered a coma was a very easy decision to make.
The things that helped - before he was completely incompetent, he signed over full power of attorney to my Mom and had her declared the person who could make all decisions about everything. He had a few sources of income and pretty much no debt, so it was a matter of managing money instead of flipping from fiscal crisis to fiscal crisis. His finances were "in order", meaning my Mom had access to all of his accounts and they were listed and organized so she could deal with his financial matters no different than her own, so that didn't take up a ton of her time. And he had his wishes known and legally documented so that when he entered the hospital the last time, it was very clear how the treatment plan should go. He was already pretty far gone before he entered the hospital but required no more care than getting him food, medication, helping him with daily tasks like going to the bathroom, and helping him find something to do with his time. But once he was requiring extensive intervention just to keep him alive, his plan was to limit care to food/water and "normal" medication and let nature do its thing. So they fixed his hip but didn't go to unusual lengths to keep him alive after he entered the coma because even if he came out of the coma he would probably be bedridden and living in what had become a pretty scary demented world for him. So he got an IV during the coma and very quickly his organs started failing and he just quit breathing a couple days later.
Things that helped his quality of life - just being around him (or having him around us), whether it was visiting him at the home or taking him out for short trips. Even if he was just slumped in the corner while everyone else had their own conversations, interacting with family/friends helped his quality of life immensely.
I got to the hospice (after flying over from England and coming straight from the airport) 2 hours before he passed, good timing I guess. I started working on the trip home less than an hour after I got the call that he was in the coma because I knew it was pretty much over. If you want to be there when he dies, be prepared to drop everything and just GO. One flight later or missed connection and I would have been there after the fact. Not so much a tragedy for me, but for my Mom who needed some emotional support after taking care of him for so long.
If any of those things are NOT in order (power of attorney, legal permissions, declaration of incompetence or whatever, living trust and advanced medical directives, finances documented and organized, etc) then get on those NOW before he becomes completely unable to communicate coherently. If you wait then other people will make the decisions for him (and you) and settling his estate can take months or years instead of just a couple of weeks. If he is already incompetent or unable to make legal decisions, get it documented so nobody questions your right to make decisions for him.
