Hi Laura,
There are a few drugs which can help, but none are without side effects of their own. He should contact his oncologist (or specialist nurse if he has one allocated). Acupuncture is also claimed to help by some people. Some support groups arrange these for free or low cost for cancer patients.
Which HT drug is he on?
If it's 150mg bicalutamide, is he on tamoxifen too, and if so what dose and since when?
Tamoxifen isn't usually given for hot flushes, but I heard recently it seems to work for some people too. (It's given to prevent breast growth, mainly due to bicalutamide.)