shop boy vs shop assistant

shop boy and shop assistant both are nouns.

Word	Noun	Adjective	Verb	Adverb
shop boy	Yes	No	No	No
shop assistant	Yes	No	No	No

As nouns, shop assistant is a hypernym of shop boy; that is, shop assistant is a word with a broader meaning than shop boy:

Other hypernyms of shop boy include boy, male child, clerk, salesclerk, shop clerk.

shop boy (noun)	shop assistant (noun)
a young male shop assistant	a salesperson in a store