boyfriend vs fellow

boyfriend and fellow both are nouns.

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

As nouns, boyfriend and fellow are synonyms defined as:

Other synonyms of boyfriend include beau, swain.

boyfriend (noun)	fellow (noun)
a man who is the lover of a man or woman	a boy or man
	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