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.

We spoke with Carol Gadd -- Duke's mom and wife of legendary Steely Dan drummer Steve Gadd ... and, she tells us they haven't received a coroner's report yet, so her son's cause of death isn't official.

She says the family was told he had a heart attack -- and a drug was probably involved ... but, no one they talked to has said it was for sure a fentanyl overdose.

Carol says her son subbed in Corey's group for a few gigs after his last drummer bailed on the tour ... so, Duke and Corey definitely knew each other. But, "Dr. Corey" -- as she sarcastically calls him -- isn't qualified to claim he knows the exact nature of Duke's death.

Gadd adds Corey's posts are especially painful because official word hasn't come out ... so, sounds like Feldman's really upset the family.

As we told you ... Corey told his fans Duke died from a fentanyl overdose in an IG post -- showing pics of DG on the drums during the tour. He said Duke was "poisoned by his own struggles in life" and died far too young.