first off i am so sorry for this. its a hard thing to deal with, when a dear loved one is sick.
From the sounds of it, you cant expect a huge turn around- but again God works in mysterious ways.
I think the best thing for you, is to stay positive, visit him... help him out, read to him, bring him his favorite new paper or magazine.. make sure he's comfy. make him laugh. tell him you love him.
Just BE there for him! It's hard to get older, as see those you love around you also getting older and going through sickness or even worse.
Just keep your head up and remember death is inevitable, we all end up facing it someday... Cherish what you have now with him.