How long has it been since the surgery? If not that long, seek out an ED rehab protocol at a major institution; or copy a protocol from a major institution and follow it.
You might have misunderstood what your doctor said. The doctor likely meant that he was able to spare the nerves after surgery. All prostate surgery traumatizes (damages) the nerves. Nerves will heal and bounce back, but they are slow growing and it takes time.
Read this book for background information; it is about ED after surgery:
“Saving your Sex Life” by John Mulhall, MD
Go to this mailing list and subscribe. It is specific about ED from prostate cancer treatment and has a lot of good info:
http://www.prostatepointers.org/mailman/listinfo/pcai
Best wishes,
Gene