Duke Gadd's mother is not happy with Corey Feldman... 'cause the coroner hasn't ruled on her son's death yet -- so, she doesn't know how "Dr. Corey" can tell everyone he knows how Duke died.

RELATED VIDEO: Debbie Reynolds Dies at 84 — Just One Day after Daughter Carrie Fisher After a long struggle with drug addition, Feldman was able to get himself clean and has remained sober for ...