My father

It may be that when your dad was diagnosed, he chose to have active surveillance rather than intrusive treatment. AS is a sensible choice for some men, particularly those diagnosed with a low risk prostate cancer when they are much older and / or they don't fancy the risk of side effects. My father-in-law was 79 when he was diagnosed but he was still sexually active so he refused the usual treatments due to the risk of incontinence and impotence. He had active surveillance for 4 years.

If your dad is a very private man, he may not have wanted to talk to you about these things as they are very embarrassing, especially for older people.

Download the treatment options information sheet from the website, and ask dad whether he is on AS (regular PSA tests, an annual finger up his bottom and possibly an annual scan (which it sounds like he has recently had)). An alternative is that he was not suitable for surgery or radiotherapy due to existing medical issues or because the cancer had already escaped outside the prostate, but this sounds unlikely if dad is definitely not having ANY treatment ... are you sure he isn't having any hormones?