Hi Julie, I don't have any experience of dexamethasone or of dealing with the terminal stage of prostate cancer. Over the five or so years I have been on this site, there have been probably five or six threads where family members/carers have posted, and obviously we all offer the best support we can. There are also long running threads where the posts come to an end, and sadly one has to assume it is for the most obvious of reasons. If one is on a cancer forum, accepting death is inevitable is a good place to start.
I certainly can't offer any practical support. Based on similar threads, I think you will have to accept his life expectancy is less than six months. You need to prepare yourself for that. Quite honestly trying to keep an 84 year old man alive is going to be very difficult. If he was in perfect health in every other way, it might be possible, but ever drug goes through the kidneys and if those are failing, there are probably few drugs he can take.
I suspect the Fanta drinking (other carbonated drinks are available) is related to the kidney problem, there is no point in changing that. There is probably little point in trying to improve his diet. In fact if he said he was planning on taking up smoking Cuban cigars and drinking a bottle of whisky a day I would probably encourage it.
If he does become nasty, don't forget he's on medication and lots of pain, so you can only do the best you can.
I really don't know much about hospices, you should contact one (that may need a referral from his oncologist) they will be able to prescribe pain killers to make his life more bearable. I know he won't go in a hospice, but as posted previously they can deal with him as an outpatient.
If he needs to claim PIP (or similar benefit) he would get it immediately, but if he doesn't need the money don't spend precious days filling in benefit forms.
Good luck, you are a very kind person.