You could say he's infected with self-doubt or simply just depressed (I've never met a depressed person who was truly happy with anything, and they usually blame themselves more than their circumstances).
One symptom of such a state is that an individual may become clingy—i.e. overly dependent upon the praise of others for feelings of self-worth.
Such a man's poor view of himself may be fueled by an underlying, persistent anxious feeling that makes him so universally uncomfortable regardless of the outside world that he begins to believe himself to be defective. For example, if the story is told from his perspective, you could use a neat little trick and just describe your character as "defective"—letting the reader infer that is the character's own opinion of himself, and not the narrator's.
You could refer to his state by its early causes, a long-ago shame or deep regret he still holds onto. One standard implication of a man being shameful or regretful is that he's not truly happy with himself.