Jason Pfeiffer -- who once worked as Klein's office manager -- tells TMZ he's sticking to his story -- that he had a sexual relationship with the King of Pop -- despite Dr. Klein's recent Facebook post, calling MJ's gay affair a lie.
Jason insists, "The story is true. We had a relationship with sexual aspects, but I'm not going to make it into some grand love affair."
As we previously reported -- Klein himself had told TMZ that Michael Jackson had a gay relationship with Jason ... even calling Jason "the love of [Michael's] life."
But the good doc has since changed his story, writing on Facebook recently, "Allegations about ... Jason being Michael Jackson's lover are ridiculous. That story was made up ... "
Now, Jason claims Dr. Klein has a secret agenda to publicly discredit him -- in case Jason's possible testimony could paint Klein in an unflattering light.
Jason adds, "Klein was the one that pushed for me to release the story in the first place."