Eating fish such as salmon, mackerel and tuna that are rich in omega-3 fats can help preventing prostate cancer.
Supplementing with 200 micrograms/day of selenium (antioxidant) cuts prostate cancer risk in half. Food rich in selenium is: Brazil nuts, tuna, sardines, halibut, spaghetti.
Tomatoes and tomato products, especially tomato sauce, have a protective effect against prostate cancer, too.
All of these foods can be also, very beneficial in stopping the progression of already existing prostate cancer.
By sharing these tips with your father you'll get this topic on another level (not used like arguing) more helpful for both of you.
Best wishes!
Marija