fellow vs boyfriend

fellow and boyfriend both are nouns.

Word	Noun	Adjective	Verb	Adverb
fellow	Yes	No	No	No
boyfriend	Yes	No	No	No

As nouns, fellow and boyfriend are synonyms defined as:

Other synonyms of fellow include beau, swain.

fellow (noun)	boyfriend (noun)
a boy or man	a man who is the lover of a man or woman
a man who is the lover of a man or woman
a friend who is frequently in the company of another
a person who is member of one's class or profession
an informal form of address for a man
a member of a learned society
one of a pair