I could be wrong but I think the reason he feels hunger, pain, etc, is that the memories he has from Cole tell him that he ought to feel these things, not because he actually does need food, drink, etc. The spirit Cole has managed to manifest itself in a body that appears real and can interact with the world, just as Imshael could seem substantial, make an imprint on the grass, but it is a sophisticated illusion. When you touch him, he appears real because that is how he imagines himself, but when he wants to be invisible, he is because that is how he wants to appear at that particular moment in time. As a spirit he has the ability to control people's minds. I've been re-reading Asunder and that is how it seems to me.
The only thing that mitigates against this is the fact that his outward appearance has changed from a boy to a young man of around 20. I'm not sure why this would occur if the spirit was only making itself real on the basis of Cole's memories but I suppose it would know that people age and so that is why he does.
If you believe yourself to be like any other normal human being, then why wouldn't you age like everyone else?
I believe that the entity we know as Cole, not knowing what he was, clung to the belief that he himself was human.
Like the spirit of valor once said:
"Do you believe these blades be steel? The staves be wood? Do you believe they draw blood? A weapon is a single need for battle, and my will makes that need reality."
Cole's will made him being a human, what he thinks a human is, a reality. hence his bodily functions and aging.
At least this is what i believe to be the case.