boy vs sonny boy

boy and sonny boy both are nouns.

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

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

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