If you are considering a home made diet, I don't recommend to do this with the help of a veterinary nutritionist. Balancing diets is very precise and difficult. If it is suspected the enlarged prostate is due to cancer, look for a commercial diet that is high in protein and fat and low in carbohydrates:
https://vcahospitals.com/know-your-pet/feeding-the-canine-cancer-patientIf you are asking about human food to give more as a treat, plain boiled white meat chicken, boiled plain white rice, peas, carrots and green beans are generally all fine to give and dogs seem to like them. This is not an exhaustive list and these food are not nutritionally balanced (even combined) so they are not appropriate to feed as the sole diet.
Intact male dogs normally do have enlarged prostates so I recommend to ask your veterinarian if they suspect pathology.
I hope this helps and please feel free to post any additional questions.