Ron, man can I relate to that. My dear father-in-law died in February, 14 years after diagnosis. I was his primary caregiver. Everyone else, including natural children were increasingly busy, "he doesn't know I'm there", etc. Sundowner syndrome is rough! You'll also notice the entire population of an Alzheimers ward gets active/irritated when it's windy, stormy, etc.
My take-away from the entire process was to dedicate myself to Nursing Home reform, but that';s another story for a different place.
Believe me, even when things progress to the point where communication is difficult, dad will know you're there.
And, what's more important, YOU'LL remember every visit on a much more intense level than before.
All the best to you and your dad.
Russ