Intuitively, I feel it is due to reduced blood flow to the organ and might help if you do some pelvic
exercises to promote better blood flow .Food items which provide your daily quota of Vitamin E and zinc should generally be helpful. Some yoga exercises like ' Padmasana ' ( Google for details) combined with deep breathing exercises will promote blood flow to the penis. What ever you do, do it in consultation with your doctor and for Yoga exercises, learn it from an instructor.