Hi Jules.

I was diagnosed and treated for prostate cancer back in 2010. I was lucky that my cancer was caught early and removed surgically. Although I'm not a doctor, I have picked up some information along the way. 

Prostate cancer in older men (late 70s, 80s, 90s) is generally less serious than in younger men. It is usually slow growing, or even lethargic, and to be brutal, most men die of something else before the cancer gets them. When it does spread then there are effective treatments that can keep it controlled for many years (although it's not possible to predict how many years because everyone is different).  Hormone treatment is very common and usually very effective, and there are newer drugs coming along all the time. Although no one is happy when a cancer spreads, it seems that your father in law is in good hands and there's no reason to panic.  In case you're wondering, surgery for prostate cancer is rarely an option for men older than 70 due to the rigours of the operation, and once it's begun to spread it's often too late to operate anyway.  

Regarding your father, I would imagine that he's being treated for benign prostate hyperplasia - essentially a swollen prostate that makes peeing difficult.  Again, there are effective treatments for this, with tablets being the first treatment choice that works for most men. I admit I'm completely astonished that he's not getting regular PSA blood tests. I can only echo the sound advice of @woodworm to insist on a test. 

There are a number of charities which provide information and support for prostate and prostate cancer sufferers.  You might like to go on their websites to get more information. Off the top of my head, there's Prostate Cancer UK and Tackle Prostate Cancer.  Both fine organisations. 

I'm active in a local prostate support group. You can find a local support group near you on the Tackle website. It's well worth going, even if you're not a patient yourself.