I'm a retired Urologist. There are basically at least 10 different types of primary prostate cancer (Gleason grades 2-10 plus anaplastic). The first step would be to determine which he has which requires a biopsy (usually done in the office after a limited bowel prep, oral antibiotics and without sedation) as well as blood tests prior to the biopsy. An ultrasound probe is inserted through the rectum and needle biopsies are taken, (6-12 typically).
Depending upon those results and the overall health of the patient surgery might be considered if it looks like a potentially curable kind. That often will require X-Rays to stage the disease first. Often that is a nuclear bone scan and a cat scan. There were some more advanced nuclear tests in study trials when I retired. I don't know if those panned out.
If those results show non-organ-confined disease (obvious spread outside the prostate) then curative sugery is out of the question, but sometimes there are debulking surgeries to help with the morbidity particularly if there is significant voiding trouble. Sometimes you do a pelvic lymph node dissection for both staging and to place metal staples around the prostate to make it easier for the radiation therapy to gauge the borders. There's a lot of potential things one might do.
With the Gleason Grade cancers, as the number gets higher, the more aggressive the cancer is. Low grade ones often just require observation. Sugically incurable ones typically get a combo of hormonal therapy plus or minus radiation, and there are many forms that either of those might take, possibly including castration. Some times the really higher grade ones get chemo as well (especially the anaplastic).
If you want your character to die quickly he should have the anaplastic variety. Those typically don't respond to anything and some people believe radiation might actually speed it up. Those are prostate cancers that are so mutated away from their original parent organ that they typically stop making the tumor markers (PSA). So when the blood tests are going down, it can be a bad sign particularly if the patient is clinically deteriorating. The anaplastic are typically more locally aggressive causing urinary and bowel obstruction and direct invasion of pelvic bones. The Gleasons tend to have widespread bone metasteses, causing bone pain and pathologic fractures with marrow invlovement leading to anemia, shortness of breath etc.
What do you want for your character?