mother's boy vs son

mother's boy and son both are nouns.

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

As nouns, son is a hypernym of mother's boy; that is, son is a word with a broader meaning than mother's boy:

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

Other hypernyms of mother's boy include boy.

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