Nick Reiner allegedly butchered his parents back in December … but when he appeared again in court Wednesday, prosecutors revealed the autopsies for Rob and Michele Reiner are not ready yet … four and a half months after their murders.

The hearing was delayed and when Nick was finally brought into the courtroom, he looked thin in his yellow and blue jail jumpsuit.

Nick’s public defender told the judge they were still waiting on discovery … and the next hearing in the case was set for Sept. 15.

As you know … Nick is charged with murdering his parents, who were found brutally stabbed to death in their Brentwood home.

Nick has pleaded not guilty to the charges — which include a special circumstances allegation that puts the death penalty on the table.

He was initially represented by attorney Alan Jackson, but the powerhouse lawyer withdrew from the case back in January. Instead, Nick is now being represented by public defender Kimberly Greene.

Nick has been held without bail at the Twin Towers Correctional Facility in L.A. … the same jail D4vd‘s been transferred to after being charged with murdering 14-year-old Celeste Rivas.

Rob and Michele died from “multiple sharp force injuries” … according to the Los Angeles County Medical Examiner, who said the manner of death was homicide … but an autopsy will reveal more details.