boy vs ball boy

boy and ball boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
boy	Yes	No	No	No
ball boy	Yes	No	No	No

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

boy (noun)	ball boy (noun)
a youthful male person	a boy who retrieves balls for tennis players
(ethnic slur) offensive and disparaging term for Black man
a friendly informal reference to a grown man
a male human offspring