Kravitz will play Gaye in "Sexual Healing" (working title) -- which reportedly focuses on Marvin's life in the 80s when he battled drug abuse and depression ... before his father shot and killed him in 1984.
Marvin Gaye III tells TMZ, "The producers and directors of this film are very wrong and shameful ... [They're] trying to do a film about a low period in his life. They don't even know the whole story."
Marvin's son says he and Kravitz were schoolmates -- and remain friends to this day -- so he wants to "talk to him about why he would do this."
Marvin Jr. tells us he and other family members are meeting with lawyers to try to stop production -- and added, "I would hope [Lenny] doesn't have any idea that we are against this film being done."
We reached out to Kravitz for comment -- so far, no word back.