The objective complement renames the direct object. In this sentence, the direct object is "him". The word "him" receives the action of the verb "consider". The word "athlete" renames/modifies "him" as an athlete.
A predicate nominative is a noun that renames/modifies the subject. A predicate adjective is an adjective that describes the subject of the sentence. In this sentence, the coaches are not being described. An indirect object receives the direct object. "Him" is not being given to anyone so there is no indirect object.
Given that 'blunt' means 'not smart' and that 'mettle' means 'wit; intelligence', I believe that the statement that best paraphrases "What a blunt fellow is this grown to be! He was quick mettle when he went to school" is "This man, who was smart in school, has grown up to be unintelligent".
