Well, I finally have a name for my dad's pathology. It's
Front Temporal Dementia. He has the style relating to executive function. As the article says this means that he is unable to perform skills that require complex planning or sequencing. This means things like tying his shoes. Considering that we used to joke about how my dad only ever did two things wrong, this is a pretty radical difference. If you want to hear about that story, just ask in the comments or in person.
The next section says Some patients remain fluent with normal phonology and syntax but increasing difficulty with naming and word comprehension. Considering he gave me a lot of my love for words, language, and questioning; this is another really big deal.
It does make me sad that this is happening but at least now, I have a handle I can grasp rather than idiopathic cerebral degeneration. It gives me something to research and something to call it.
Bittersweet isn't quite the right mood since I want a word meaning sadness tinged with happiness rather than vice-versa. I know there is a word meaning this and I know there is a term reflecting that word's relationship with bittersweet but I can't think of either of them.