son vs mother's boy

son and mother's boy both are nouns.

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

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

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

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

son (noun)	mother's boy (noun)
a male human offspring	a boy excessively attached to his mother; lacking normal masculine interests