Doctors won't give odds or a prognosis because it is so incredibly variable. You already sense that this is not going to turn out well, or you would not be seeking answers like this.
I've been through something similar with my dad, who did pass away in the hospital. However, at many times in his situation, he had a small rally and was doing better. This is normal. Had one of his rallys 'stuck', he probably would have walked out of there.
Overall health and other factors come into this, but the patient's morale is a big issue. The better he feels, the better his chances.
It is smart to be ready for the worst, but he should feel like this is just a minor blip that will be over in a while.
Try things to lift his spirits, without putting too much stress
on him. Nice music he likes playing in the background, pictures that lift his spirits (photos or drawings) placed so he can see them easily, etc.
If I had it to do over again, I would work on all of his senses- sight (pictures), sound (music), scents (potpourri oils if allowed), touch (gentle massages, caresses, snuggly teddy bear), time (one complaint of many patients is the lack of clocks. A small clock off to one side sometimes helps minimize the 'twilight zone' effect), etc.
Whenever he was strong enough, I would use humor- the latest jokes from a website, a funny DVD, an audiobook of a comedian's routine, etc., and comfort foods- even if only small portions- of favorite candy or treats, or home-cooked foods.
Now- having said all that, please be aware that often the person who needs the most attention from other family members is the spouse or primary caregiver. It is all too easy for that person to become horribly depressed and exhausted. While the patient needs help, remember that other people do as well.
Good luck, and god bless.