Hormone therapy seeks to reduce the amount of testosterone in a man's body because prostate cancer usually needs testosterone to 'feed' it.
Conventionally it's given by regular injections (at three-weekly intervals in many cases) or sometimes by oral drugs. Occasonally the surgical removal of a man's testicles is used to ensure that no testosterone is produced.
However there was clinical trial running for about a decade where those who agreed to participate were randomly selected to either have three-weekly injections as normal or to have oestrogen patches stuck on their limbs instead. (The idea is that using oestrogen patches doesn't increase the risk of osteopirosis, as long term use of the injections can do).
I applied to go on the trial and, as I'd hoped, got selected to try the patches. (They're exactly the same patches that women use for HRT treatment). I started off with having two on each of my limbs but then, based upon satisfactory blood tests, I was told that I only needed to use three patches instead of four. I simply change them twice a week, on Tuesdays and Fridays. I've experienced no major side effects from them (although it would be hard to separate out any of their side effects from those produced by the chemotherapy) other than I appear to be growing breasts! My testosterone level though is a big, fat zero.
The last date for signing up for the trial (which only gave you a 50/50 chance of getting the patches anyway) was the 30th of April this year so, unless (based upon the trial results from people who started using them a decade ago) they've already been accepted into mainstream prescribing, your FIL probably wouldn't be offered them.
However there might still be some benefit from him being given the injections or tablets though.
https://www.cancerresearchuk.org/about-cancer/prostate-cancer/treatment/hormone-therapy