The Chicago Tribune said the office also ruled Julian King's death a homicide after conducting an autopsy on his body.
Citing the ongoing police investigation, officials didn't say where the child's wounds were, how many times he was shot or how long he had been dead.
The Tribune said a visibly shaken Hudson clutched the hands of family members Monday when she visited the Cook County morgue, saying, "Yes, that's him," as she was shown video images of the second-grader, whose body was in an adjoining room.
The Oscar-winning performer had previously been to the morgue to identify the remains of her mother and brother, who were shot to death last Friday. Her nephew's bullet-ridden body was found early Monday in an abandoned sport-utility vehicle parked on a Chicago street.
Police were questioning William Balfour, 27, the estranged husband of Julian's mother, Julia, in the slayings, but no charges have yet been filed.
He is being held on a parole violation.
A police source told the Tribune the motive for the slayings could be related to disputes between the suspect and his estranged wife and her family.
Police also say it is possible more than one person was involved.