male parent vs old man

male parent and old man both are nouns.

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

As nouns, old man is a hyponym of male parent; that is, old man is a word with a more specific, narrower meaning than male parent:

Other hyponyms of male parent include dad, dada, daddy, pa, papa, pappa, pop, father-in-law, pater.

male parent (noun)	old man (noun)
a male parent (also used as a term of address to your father)	an informal term for 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