old man vs male parent

old man and male parent both are nouns.

Word	Noun	Adjective	Verb	Adverb
old man	Yes	No	No	No
male parent	Yes	No	No	No

As nouns, male parent is a hypernym of old man; that is, male parent is a word with a broader meaning than old man:

Other hypernyms of old man include dysphemism, begetter, father.

old man (noun)	male parent (noun)
an informal term for your father	a male parent (also used as a term of address to your father)
a man who is very old
a familiar term of address for a man
(slang) boss
aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe