Quincy -- son of the legendary music producer -- just filed legal docs, responding to Lil Wayne's copyright infringement lawsuit against him, in which Wayne claims he never gave Quincy permission to use his music in the film.

It didn't help that Wayne also HATED the movie -- calling it a "scandalous portrayal" of him -- and then gave Quincy's lawyer hell in a hilarious deposition.

But Quincy's insisting he had the right to use several Weezy songs in the documentary -- including "Lollipop", "A Milli" and "Don't Get It" -- under the "fair use" doctrine because the music was necessary to tell the Lil Wayne story.

Quincy wants Wayne's lawsuit dismissed asap. He also wants Wayne to pay his attorney's fees.