Good luck now there is a defined path forwards. I hope that they will be able to keep the hormone treatment to the minimum necessary to optimise the radiotherapy, and use a drug which gives the best chance of testosterone recovery afterwards.
You are going to have to manage things as they come. In my case I never had hot flushes, whereas for others they are a major issue. I assume the masturbation advice is to keep erectile function intact (the catchphrase is "use it or lose it"); basically even if the hormones are a mess the penis may respond to tactile stimulation, which could be from you as well as him.
Diet is an interesting question, I don't know whether anyone knows whether it helps with navigating hormone therapy. But it is good for anyone, and certainly anyone with a known health problem, to eat healthy food and avoid ultraprocessed stuff. Until... the radiotherapy starts, when you are likely to have the problem of your husband adjusting diet to ensure an empty rectum during sessions. The hospital will no doubt advise on their preferred routine, and if you search previous threads here you will discover other people's experiences of that.