Paralysis from prostate cancer would usually occur because of a fracture on the spine caused by a bone metastasis. For most cases, this would be preceded by pain, so unless he is on pretty strong analgesics, it is possible that he never felt the pain in the back until the bone breaks and he would thereafter have no more pain as nerves get injured.
If you have some preliminary tests like a PSA, a reading of more than 20 would raise the possibility of bone metastasis (that is if there is evidence of prostate cancer verified from a biopsy – some patients with prostatitis may have PSA well beyond 50 and yet have no cancer).
Have only just read your comment about your father - my husband has prostate cancer, and secondary bone cancer. Directly after radium therapy he suddenly lost the use of his legs, and the way in which you explain your father's paralysis sounds exactly the way it affected my husband. I feel that the radium treatment could definitely have played a part in this. He has been paralysed for a year now, and there are no signs of recovery, although he is excercised and massaged daily.
my dad was 71 years old and he died last week on a wednesday, my dad was very strong and very active...that is until one afternoon he just fell to the floor because he suddenly could not feel his legs any more, we rushed him to the hospital and for a whole month they gave him therapy for his legs and nothing no progression he started having pain on his legs, and lost his appetite, they ran test on him gave him an MRI etc, found out my dad had mestatic disease, and it was spreading....i still have unaswered questions because he never mentioned he had any of this and we lost him so fast...so please ask alot of questions and be by his side.....thats what we all tried to do =(