I’m afraid if you are looking after an older person you have to have an eye on the endgame and that might involve going into a care home.
It’s best to be open about this and have a discussion with the family, and himself if he is capable.
Have you got all the relevant medical information? Has he been tested for dementia and other issues? All this would be important for that discussion as well as this:
Do I need specific documentation or anything?
You really need some kind of Power of Attorney but his cognition may have declined beyond the point he could consent to this. It would be worth getting legal advice though.