Don't panic! A diagnosis of 'Prostate cancer' covers a wide spectrum, from 'only just' to 'really developed'. He will (should!) now undergo a series of tests to determine how extensive / aggressive the cancer is and where it is - MRI, bone scan. Once you know this, the doctors can come up with proposals for treatment. These will range from 'active surveillance' (i.e. watch and wait) to prostatectomy or hormone treatment / radiotherapy to brachytherapy. Generally speaking, prostate cancer is slow to develop and spread, and responds well to whatever treatment he chooses. Most men over the age of 65 have it. My oncologist once told me that it is something which men die with, rather than of. It is normal that you should be concerned, but at this stage, try to keep calm, listen to the advice / information coming from the specialists, and tell yourself / your father that he will beat it.
best wishes for some good news,
Hermit