sonny boy vs boy

sonny boy and boy both are nouns.

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

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

Other hypernyms of sonny boy include male child.

sonny boy (noun)	boy (noun)
a male child (a familiar term of address to a boy)	a youthful male person
	(ethnic slur) offensive and disparaging term for Black man
	a friendly informal reference to a grown man
	a male human offspring