I think because your dad doesn't exhibit the usual signs of dementia and because he is deteriorating slowly, then it is harder to accept the dementia diagnosis.
My husband was diagnosed over five years ago with 'atypical' Alzheimers which meant that he didn't fit the usual pattern of the disease. Then it was 'frontovariant ' Alzheimer's but he didn't really fit in there either. The neuropsychologist says he has 'nonamnesiac' Alzheimer's but the geriatrician says he has a 'non classical ' form of the disease.
So OH doesn't behave like other dementia patients. His long term memory is shot and he can't recall much of the first thirty years of his life. His short memory can be a bit fuzzy at times and he can't use a mobile phone, drive a car, do online banking etc. However, he is still continent, showers without help every day and can still prepare simple meals. He still plays bridge several times a week.
Please don't be too hasty in denying your dad's diagnosis. There are many, many forms of dementia and research is still discovering more. I am pleased that your mum is so supportive.
When my dad told me he'd been diagnosed with mixed dementia I was devastated
But what's made me start to think he was possibly misdiagnosed is he's always acknowledged he's forgetful and it's usually things like what he's eaten during the day, birthdays or occasionally names (our family is very large) - all things he'll remember if he's given a clue or a reminder. He's also still capable of recalling things that happened 70+ years ago very accurately
He knows who his family/friends are (even though he hasn't seen most of his family in years) and can easily recall things like his date of birth, our address and telephone number without any difficulty. I've been living with him for nearly five years now and
at the moment I don't think he's showing any of the other symptoms of early stage dementia which I've researched. He has been housebound for a while (he had to stop driving due to his diabetes) so I have considered he may possibly have depression
But of course I am prepared for the fact he may have dementia and it's just taking a very long time - to be honest what I think I'm doing is trying to put the diagnosis to one side because he's only slightly forgetful at the moment. If he gets worse then he does know I'm here to support him for as long as I possibly can. My partner is really supportive too even though we haven't been together for very long
I'm really sorry to hear about your husband's diagnosis. Do you have any support from your family/friends?
I do really appreciate your kind words though. They have helped