Father's sudden decline. Anyone else experienced this?

My father who has numerous health problems and a family history of alzheimers has declined over the last year. He has had out of control diabetes until recently, COPD and sleep apnea. However , over the last year his short term memory has all but disappeared entirely. He becomes upset very easily and knows something is wrong. I don't know that he understands it cannot be reversed. It happened so rapidly and now some long term memory is becoming affected as well. We have seen numerous doctors and a new neurologist that put him on some medication that so far is not helping. He has also started to have problems sleeping and horrible nightmares. I'm shocked at how quickly he declined and went from a financial genius to someone who didn't know what to give the accountant for his taxes. Has anyone else experienced a a rapid onset and decline like this with someone who has dementia/alzheimers? We have had imaging done and it was all clear just normal deterioration that was age related. He is only 69 and I'm concerned I'm losing him very quickly. Its affecting his entire life and he says he doesn't want to continue like this and we are trying everything.