Hi Maggie,
my case has some similarities to your husband's in that the cancer was too far advanced at diagnosis for an operation (if I've read his bio correctly) and I also had multiple mets in my pelvis, spine, ribs etc.
I too had 6 cycles of Docetaxel but this was quickly followed by 18 sessions of radiotherapy targeting the primary tumour in my prostate, which also helped to further shrink my various mets.
I found the radiotherapy to be quite easily tolerable, but the caveat is I was 55 at the time. I felt a little tired towards the end of the 18 sessions, but I found that the side-effects were by no means as bad as from the Docetaxel chemo. The biggest problem was having to go to the hospital 5 days a week for nearly 4 weeks.
I wasn't having incontinence issues before the RT, nor did I have any afterwards. I was warned of the possibility of incontinence but it didn't happen.
I would ask the consultant how many sessions of RT would be needed for your husband. If it's only a single session or a handful of sessions, my 10p worth would be to try it.
Best of luck whatever you decide, and please do ask me if you've any questions!
Craig