sonny boy vs lad

sonny boy and lad both are nouns.

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

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

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

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