I am not certain exactly what is occurring. The cause for the urinary symptoms is unclear given the normal urodynamic test. However, given the exclusive symptoms in the lower extremities the problem may be in the distal spinal cord or the individual nerves. The normal MRI excludes most abnormalities. He may need to have an EMG (muscle/nerve test), blood tests for inflammation, lyme disease, and connective tissue diseases, and then perhaps a spinal tap. The spinal tap is to look for inflammation within the spinal canal. I hope this helps. Good luck.
Don't let him give up! If he is having injections, he is obviously experiencing the pains that go along with a ruptured disc. He needs to go back to his surgeon and tell them he is not satisfied with the findings. He is the one in pain and having the problems. He knows how he feels and he needs to be persistent until he is satisfied. My nuerosurgeon thought it was something else when I needed emergency surgery which I finall received 5 months too late. Good luck!