son vs Jnr

son and Jnr both are nouns.

Word	Noun	Adjective	Verb	Adverb
son	Yes	No	No	No
Jnr	Yes	No	No	No

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

Other hyponyms of son include Jr, Junior, mama's boy, mamma's boy, mother's boy.

son (noun)	Jnr (noun)
a male human offspring	a son who has the same first name as his father