No one knows what will happen next. I sat with my grandfather while he was in his final stages and share the feelings of loss and helplessness that you are experiencing now. In terms of Parkinson's and Alzheimer's diseases, prognosis is only a guess. We know pathologically what is happening in the brain, but how it will affect each and every person varies as much as each person varies from another. We believe to this day that he chose to die while we were gone to save us the discomfort.
Life is certainly unusual and the end of life is no exception.
These are tough times for all. I hope you have others who are there for you, as you are there for them. He is probably frightened and confused. However, as many people as I have seen in the final stages of their lives, the body and brain seem to prepare for the event in miraculous ways. But in my opinion, life is just too precious and complex for it to end with only a few years on earth.