old boy vs old man

old boy and old man both are nouns.

Word	Noun	Adjective	Verb	Adverb
old boy	Yes	No	No	No
old man	Yes	No	No	No

As nouns, old boy and old man are synonyms defined as:

old boy (noun)	old man (noun)
a vivacious elderly man	an informal term for your father
a former male pupil of a school	a man who is very old
a familiar term of address for a man	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