I’m so sorry you’re going through this. The same thing happened to my mum last year - she was suffering from dementia but was physically fine, and a very minor fall resulted in hospitalisation, which then escalated very suddenly and unexpectedly into terminal care.
As others have said, staff working in these settings are usually very good at assessing and managing pain and discomfort. They’ve seen the arc many times before and understand the rhythms of EOL care.
We were extremely distressed when my mum stopped eating and drinking, but were reassured by staff that this is a natural part of the process. It feels awful and neglectful because we can’t imagine going without water, but a change in the brain chemistry of terminally ill people means they no longer require sustenance - and also that they avoid the risk of inhaling fluid and choking or suffocating, which would be a far more terrible and frightening way to go.
It’s very, very hard on you to witness your dad going through this, and the shock of unexpectedly being thrown into this situation will magnify your anxieties, but provided he’s not suffering and he knows you’re there, it can be a positive time for all of you. You can talk to him, hold his hand, play music he enjoys, just sit quietly. I now treasure the time we were able to spend like this with mum. It gave us a bit of space to adjust and accept and be grateful. She knew we were there and her passing in the end was very gentle and peaceful. She did not suffer.
I hope you and your family are able to find a way through that feels right for you. Sending you a big hug x