Steven, Have you seen a Urologist? Have you followed up with your doc about this? I work at a Urology office but not sure if you have tried any help from docs.
Hello,
What you are having is an erectile dysfunction. The causes are many like diabetes, high blood pressure, heart or thyroid conditions, poor circulation, low testosterone, depression, spinal cord injury, nerve damage (for example, from prostate surgery), or neurologic disorders (such as multiple sclerosis or Parkinson's disease).
In your case, nerve damage after the back surgery may be responsible. Consult a neurologist and get a NCV (nerve conduction velocity) test done to confirm the diagnosis.
I sincerely hope that helps. Take care and please do keep me posted on how you are doing.