People with dementia lose logic and the ability to follow reasoning. Once they get an idea into their heads you cannot use reasoning to correct this idea. My mum was certain that people lived above her (who it was changed, but she was always convinced there was someone) who were doing things like pinching all the hot water or rearranging the furniture, despite living in a bungalow! At the beginning I tried to reason with her that there was nobody there. She would agree that she lived in a bungalow and yet
at the same time insist that people lived above her. I tried taking her round her bungalow to show me the stairs and took her outside to show her that there was nothing above her, but nothing worked. She did not believe me when I said that she was wrong because she
knew that she was right!!
It sounds like your dad is sundowning and when that happens many people with dementia interpret the feeling of increased confusion and anxiety with feeling unwell. My MIL used to phone me up most evenings (multiple times) to tell me that she was so ill, although doctors never found anything wrong. I suspect that this is what is happening to your dad and he interpreting this feeling of being unwell as being a blood sugar hypo.
I agree with
@Canadian Joanne , that the way to go is to try and reduce his anxiety. An antidepressant/anti-anxiety medication could help a lot.