As I understand things:
In general, "dementia" is a term used to describe symptoms , in this case a progressive loss in cognitive functions.
There are many possible causes of dementia.
Alzheimer's (Disease) is a specific condition, with a specific underlying physical cause, which causes the symptoms of dementia and which is diagnosed through the symptoms presented, cognitive tests and the elimination of other possible causes (such as a thyroid condition).
I believe there is no way to physically and definitively diagnose it other than an autopsy and examination of the brain.
So dementia is not a specific disease as such but more a descriptive term.
It's a bit like saying "that person has a spotty rash" as opposed to "that person has measles". Both are equally valid.
Alzheimer's Disease is by far the most common cause of dementia.
Just to add to the confusion, it's possible for a person to have more than one of the diseases at the same time.
Also, some of the diseases have "dementia" in their names, for example, Vascular Dementia.
They all have symptoms in common, but usually progress in different ways, with some symptoms being rare with one disease but common in another.
For example, Alzheimers is usually marked by a slow and gradual decline, whilst people with Vascular tend towards periods of stability and sudden occasional declines.
However, every person is unique, it's almost impossible to say how things will go or what will happen, or when.
It might be worth your asking your dad's GP or specialist for more information.