Hello, and welcome to the forum --
Not all people reach stage five, but I have been told that symptoms are often more severe in the elderly, so perhaps that could account for the speed of his case. Also, getting the right combination of medications is crucial. It often takes a lot of experimentation, so be sure your grandfather is seeing his doctor or Parkinson's nurse frequently to recheck symptoms, monitor progress, etc.
He is fortunate to have a grandchild who cares enough to research his problems and their treatment. I'm sure someone on this forum more knowledgeable than I will answer soon and offer a more valuable response.