Urinary retention after spinal anesthesia or specifically after sympathetic nerve anesthesia can be due to impaired motor function in the regions supplied by the nerve. The bladder is more sensitive to complications, such as retention following anesthesia with narcotics.
As your father is able to feel the nerve sensations that indicate bladder filling, his sensory nerve supply seems normal. The problem, then, may lie at the sphincter or detrusor level. The detrusor muscle of the bladder is part of the process of urination and if motor nerve supply to this muscle or the sphincter is impaired there would be no or inadequate contraction (detrusor) and relaxation (sphincter) required to empty the bladder. This can occur as detrusor areflexia or detrusor sphincter dyssynergia.
You can ask your doctors to prescribe some medicines, such as bethanechol (cholinergic agonists), which have been known to help relieve urinary retention caused by detrusor areflexia.
Additional measures include increasing ambulation by the patient, external pressure on the bladder by pressing hands over the lower abdomen (done by the patient himself) on feeling the sensation of bladder fullness, pinching (lightly) the dermatomes of the lower back as these stimulate the detrusor. Improving his intake of vitamin B complex and other micronutrients will also aid faster return of function.
Do keep us posted on any additional doubts.