Cause may be so many..proper case history will be good for suggesting anything more appropriately..
you can eat raspberries,blueberries,milk with turmeric,fenugreek seeds with lukewarm water,ginseng,garlic ,green celery,asparagus,dark chocolats,drumsticks,avocados,walnuts..
Massage penis with seasme oil ..
Go for a homeopathic treatment..