boy vs mother's boy

boy and mother's boy both are nouns.

Word	Noun	Adjective	Verb	Adverb
boy	Yes	No	No	No
mother's boy	Yes	No	No	No

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

boy: a male human offspring
mother's boy: a boy excessively attached to his mother; lacking normal masculine interests

Other hyponyms of boy include Jnr, Jr, Junior, mama's boy, mamma's boy.

boy (noun)	mother's boy (noun)
a youthful male person	a boy excessively attached to his mother; lacking normal masculine interests
(ethnic slur) offensive and disparaging term for Black man
a friendly informal reference to a grown man
a male human offspring