cover boy vs adult male

cover boy and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
cover boy	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of cover boy; that is, adult male is a word with a broader meaning than cover boy:

Other hypernyms of cover boy include man, photographer's model.

cover boy (noun)	adult male (noun)
a very pretty boy who works as a photographer's model	an adult person who is male (as opposed to a woman)