Sounds like my baby brother (he's 54), except he has fewer brain mets. He was a mess right after the radiation -- mentally out of it, barely walking; then he had some neurological symptoms and seizures and spent a week in the hospital. He was so obnoxious that I was thinking "He won't die of cancer. It'll be blunt force trauma, because I'm about to take off my clogs and pound those metastatic lesions right out of his head!"
Between the treatments and the medications, managing the side effects is a delicate balance. You can't tell what's the disease, what's side effects and what's emotional stress.
Two weeks after hospitalization, he's a lot better, but obviously still sick. He's a little depressed after making the decision to retire; I'm hoping he will be encouraged by paperwork that seems to assume that people who retire on disability are probably faking it. They keep telling you what you must do if you go back to work, or else...
It seems to me from what I've been reading that when the medical professionals start talking months, what they mean is "This is how much we can give you. It's up to you, now." With identical diagnoses, some people get up off their deathbeds and live until they get hit by a bus, and others turn in their resignations and "move on" two weeks later.
A lot depends on your dad's personality. If he's able to interpret I can't be left alone as everybody has to come over and watch the game with me instead of everybody thinks I can't do anything anymore, he'll do better. For my brother, I'm starting to think that maybe those two crazy cousins that nobody tells when they move might be a good diversion; maybe I'll give them his address next time they call...
That's one perspective for you.