The oncologist knows you have cancer from the results and his experience so far, but does not know how aggressive is the growth, the biopsy results, or it's progression, the body scan.
Your psa is increased so it's a no brainer for the medic to prescribe HT instantly to try and slow the growth and maybe reduce the cancer.
Not until all the results are back will there be a more clear picture of the problem and the extent of treatment needed.
Advice is listen to the oncologist, with your family member present, discuss treatment options these may vary due to aggressive nature of the problem, age, what's available locally etc. Do some research on the available options. Discuss these again with the oncologist, agree a treatment plan giving you the best outcome for your situation then you can do no more but put yourself in their hands as there is little choice but to get on with it.
I found even though I was a wuss that there is a certain calm about having no further choice and hand myself over to them to get on with it. I laugh now to think I used to be scared of needles lol...