TMZ broke the story ... Kelly bullied 27-year-old Lenny Felix into singing "I Believe I Can Fly" and then mocked him endlessly after a chance encounter outside a Hollywood nightclub. Kelly's since taken down the video in which he ridicules Lenny and replaced it with an apology video.

But the family's spokesman, Lenny's uncle Scott Lettieri, tells TMZ ... the apology was insulting because Kelly "would not take responsibility for disrespecting and getting a laugh at the expense of a sensitive young man who suffers from autism."

Lettieri went on to say Kelly used "Trumpian rationalization for shaming and bullying a sweet autistic young man ... [and] we feel there is no recourse but to pursue legal action."

We've reached out to Kelly, he had no comment.