man-child vs boy

man-child and boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
man-child	Yes	No	No	No
boy	Yes	No	No	No

As nouns, boy is a hyponym of man-child; that is, boy is a word with a more specific, narrower meaning than man-child:

Other hyponyms of man-child include son.

man-child (noun)	boy (noun)
a child who is male	a youthful male person
	(ethnic slur) offensive and disparaging term for Black man
	a friendly informal reference to a grown man
	a male human offspring