Hiya, my husband Bruce, has myeloma and kidney failure (he has dialysis 3 times a week).
He has to be careful about his fluid intake because of 'hidden fluids'. Dialysis removes excess fluid as well as cleaning the kidneys. He has to watch what he's eating as well as how much he drinks. Jelly, icecream, salad, custard, soups, stews all contain a lot of liquid. Maybe getting your Dad to eat some of the things my husband has to avoid will help keep him hydrated.
Cheers
Mal & Bruce.