I would ask your dad why he does not like to increase his fluid intake.
It could be something to do with going to the toilet. Is he able to go to the toilet alone,
Is he embarrased,
Does not feel safe?
Does not like to bother people, feels a burden?
Sometimes elderly people can be a little stubborn. They are at a stage in their lives when help is needed this is hard for people who have always been in control.
It maybe this stubborness or refusal to drink fluids is an attempt to take back sense of control.
Lots of things may be contributing to him not taking more fluids try not to dismiss the fact he is wheelchair/bed bound and drinking more means more trips to the loo.
Can a pd nurse have a chat with him about how he feels ,avoiding lectures .
I had a similiar problem with my mum stubborn , I backed off just reminded her that to reduce risk of infection had to have 2 litres liquid a day. Just over 4 pints?
My mum drinks loads now it was her idea to increase fluids it was important to her not to be treated like (in her words) a child. Backing off helped in my case.
In the meantime I made jelly, blancmange, caramel puddings , egg custard, trifle all lots of liquid. Ice lollies, ice cream and soups. I was sneaky too.
Good luck and take care x