The only way you go from nothing to metastatic cancer is if you don't look for it. Given that almost every man over 50 gets a PSA at least once a year, it's impossible they missed the prostate cancer. Impossible. You don't even need a biopsy or imaging, just the blood test. Once those numbers go north of 4, 4.5, it's game on with all of it: biopsy, imaging, surgery, chemo, radiation. And also given that those numbers trend high and don't go from 1 to 4.5 quickly and that metastatic disease is a result of long-term disease, he's had it more than a minute.
I don't know that dementia is related to treatment. I think his mental decline is parallel to, not because of, his cancer.