The character is a man in his 50s with several pre-existing chronic conditions, but nothing that could cause death. I want him to come down with something that will kill him in 6-12 months maximum, is not genetic, is preferably rare, and is an excruciating experience. I would honestly rather stay away from cancer if possible. I can shorten the timeline by a lot (down to weeks) if possible.
I considered CJD and sporadic fatal familal insomnia, as well as various types of cancer.
Various google terms, including but not limited to 'deadliest kinds of cancer', 'rare and painful diseases', 'creutzfeldt-jakobs disease', and 'diseases to kill in 6 months'.