Effectiveness of Zoladex and Casodex on Metastatic Prostate Cancer

I am seeking guidance on how long Zoladex and Casodex are effective towards slowing the spread of metastatic prostate cancer.

My father is 85 and was diagnosed with prostate cancer approximately eight years ago.  The cancer has now metastisized to his hip, spine, ribs, arm and shoulder.  When first diagnosed he took Zoladex and Casodex but discontinued the Zoladex after two years.  To my knowledge he has been taking Casodex since his original diagnosis.  Six to nine months ago his urologist convinced him to begin taking the Zoladex shots again.  Within the last two months he has undergone paliative radiation treatments which have been very successful.  The radiologist told us that he doesn't think the Zoladex and Casodex are helping, however we have not yet been able to consult with the prescribing urologist and oncologist.  My father had a bone scan three months ago and it did not show any new spots as compared to the bone scan taken a year or two earlier.  Some spots had improved and some had gotten worse but the scan did not show any new spots.  

I understand that Zoladex and Casodex eventually stop working however if the bone scans are so similar could these drugs be keeping the cancer from spreading as quickly as it might if not taking the drugs?

Thank you in advance for advice on this issue.  While my father is 85 he has been in good health until recently and his siblings have all lived to their mid-to-late 90s.

Janet R B