My father refuses to wash more that once a week. I have to stand in the doorway and explain, "what to do next" while he is in the shower. I've tried to get him to do it more often but he out and out refuses. I tell him I will have to get someone to do it for him if he can't do it anymore and that just makes him angry. I am exhausted all the time from explaining everything to him. He has dementia and can't remember things 5 minutes after you tell him. Lots and lots of repetition.
When showering a dementia client. I wear a swimsuit and get in the shower with them. With a warm room and a chair it can sometimes be enjoyed.
By warm room I mean at least 80 degrees. That cold blast getting out of the shower will be what gets remembered rather than the enjoyable head massage.
Once a week to shower is fine.
Place dad in Memory Care Assisted Living if you cannot handle him at home. Or hire in home help on his dime. Things only get worse as dementia progresses, never better unfortunately. You'll be repeating yourself constantly because dad has no short term memory, meaning he cannot properly function any more.
I recommend a book called Understanding the Dementia Experience by Jennifer Ghent-Fuller available on Amazon for you to read. The 36 Hour Day is another good book to use when you have a question about a specific thing relating to dementia.
Good luck to you.