The phrase "a little fellow" is correct and usable in written English.
It can be used to refer to a small or young person, often in an affectionate or endearing manner.
Example: "The little fellow ran around the playground, laughing and playing with his friends."
Alternatives: "a small child" or "a young lad".
It is a mighty claim for a little fellow.
"Such a little fellow -- his file seemed bigger than he was," she said.
But about three minutes after I had entered the apartment, the bell rang and a little fellow who looked like a jockey was standing there.
He's an innocent abroad, a Little Fellow who loves but whose girl is also pursued by a man who's twice his size and prone to aggression.
Back home in Czechoslovakia, while still a little fellow, he served as an apprentice instructor & went into the business on his own as soon as he qualified.
(14 December) Bilbo Baggins is a little fellow from a western nation who journeys east, through great danger, on a mission to steal a dragon's treasure.
So schedule a play date, pour some coffee and enjoy a little fellow-mom therapy.
"He was a modest little fellow, but had a peppery temper and could say some cutting things when anyone displeased him," Senator Bernard F. Martin said a few days after Hall's death.
Undoubtedly heterosexual, he was a washout at romance — no amorous Walt Whitman or even the intriguingly neurotic Thomas Eakins but "a quiet little fellow," in the words of a friend — numb to personal magnetism, female or male, and with a pronounced distaste for bodies except in action.
The characterization of Billy's best friend, for instance, a flamboyant little fellow who seems to be channeling a disco queen from Studio 54, feels completely ersatz to me.
Mr. Gordon "was a quick little fellow," said Irving Fein, Benny's manager, in a 1998 interview for the Archive of American Television.
