sonny boy vs sonny

sonny boy and sonny both are nouns.

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

As nouns, sonny boy and sonny are synonyms defined as:

Other synonyms of sonny boy include cub, lad, laddie.

sonny boy (noun)	sonny (noun)
a male child (a familiar term of address to a boy)	a male child (a familiar term of address to a boy)