It is a somewhat difficult question, because it is an ethical question. Fifty years ago it was considered acceptable for people who new what is best, to make decisions on behalf of people who wouldn't have made the right decision themselves. If you look at forced adoptions, forced sterilisation, experimentation on servicemen you can see that it is not a good route to go down.
Of course we don't allow children or the insane to make important decisions, probably with some justification.
If your dad is not informed he cannot make decisions about HT which is a possible treatment, also if he knows his life is limited he may want to spend it on holiday, or chasing fast cars and fast women; good choices, but ones he can't make without all the information.
You consider a cancer diagnosis may impact his life so badly that it outweighs his right to make informed choices. If you know the diagnosis and he doesn't you will have to continue making choices on his behalf without his consent.
The following is my personal opinion. If he can't know his diagnosis I wouldn't want to know it either, because there is no way I would want to be making decisions for him or preventing him making informed decisions. I would want him to stop the diagnosis process, now.