Lupron not shrinking prostate?

Hi all, my father was prescribed Lupron after his PCa diagnosis this past November (Gleason 9 & 10, seminal vesicle involvement, no other spread detected), partly in an effort to stop the cancer, but also to shrink the prostate before radiation scheduled for late March.
It seems to have succeeded in lowering his test. level, but after 2+ months, it doesn't really seem to have reduced the size of the prostate.
How long does it take to shrink it? Would it be noticeable? It is quite large at about 100 grams extimated size, pushing into bladder, and causing a lot of pain.
Are some PCa's resistant to hormone therapy right from the start??