To be honest, he has just been diagnosed with cancer - I would let him have a drink if he wants one. There is very little if any reliable research to suggest that cutting out alcohol is going to make any difference to his prognosis. The same goes for food - apart from some snake oil salesmen & quacks, there is no evidence that your dad changing his diet will help with the cancer. There is lots of research about cutting out red meat, dairy, processed foods, etc and eating more garlic, onion, processed / cooked tomatoes, oily fish but, and it is a big but, the research indicates that these diet changes need to be made when men are still boys - they don't help with prostate cancer once you have it.
Having said all that, there is no doubt that eating more healthily benefits everyone so cutting down on sugars is a good idea. If he is interested in this area, I can recommend the Prostate care cookbook by the Prostate Cancer Research Foundation - it is available on Amazon, has a range of easy to follow recipes and explains the research.