young buck vs young man

young buck and young man both are nouns.

Word	Noun	Adjective	Verb	Adverb
young buck	Yes	No	No	No
young man	Yes	No	No	No

As nouns, young buck and young man are synonyms defined as:

young buck (noun)	young man (noun)
a teenager or a young adult male	a teenager or a young adult male