I think it is pretty straightforward to judge the Father's dreams as nostalgic. What he thinks of are times before the cataclysm that has changed the world as we know it, and he recalls moments of intimacy with his wife that clearly show his desire to return to those times, before the life and death struggle that life for him has become. Consider the daydreams that he has towards the beginning of the story as he and the boy endlessly walk on together:
From daydreams on the road there was no waking. He plodded on. He could remember everything of her save her scent. Seated in a theatre with her beside him leaning forward listening to the music. Gold scrollwork and sconces and the tall columnar folds of the drapes at either side of the stage. She held his hand in her lap and he could feel the tops of her stockings through the thin stuff of her summer dress. Freeze this frame. Now call down your dark and your cold and be damned.
Note the way in which he attempts to remember happier times with his wife, before she chooses to end her life. However, in a sense, this nostalgia is potentially dangerous as it threatens to distract him from the grim and serious battle for survival that he must, of necessity, engage in to save his son's life. Thus there appears to be no doubt that the Father's dreams are not nostalgic.