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.

Corey Feldman. “She was just very supportive and very positive and sweet. Every minute on the set with her was a pure joy, honestly,” he tells PEOPLE. “We never had an argument; we never had ...