Here are the rules regarding juvenile records for the state of Missouri.
Think Before You Plead: Juvenile Collateral Consequences in the United States
Here's one snippet that makes me think he wasn't convicted of second degree murder:
......"Juveniles accused of committing a crime and subsequently found guilty are considered adjudicated delinquent (adjudicated) rather than convicted guilty (convicted).[6] With a relatively few exceptions, the juvenile courts have exclusive original jurisdiction over children under seventeen who are charged with acts that would be crimes if committed by an adult."...........
.........."Public: Juvenile records are not open to public inspection, except by a court order allowing a person with a “legitimate interest” to view the records.[28]
However, if a child is adjudicated for an offense which would be a felony if committed by an adult, the records of the dispositional hearing and proceedings related will be open to the public to the same extent as adult criminal proceedings.[29]However, the social summaries, investigations, and status reports submitted to the court by any treating agency after the dispositional hearing are kept confidential and open to inspection only with approval of a juvenile court judge.[30]Additionally, peace officers’ records on a juvenile are treated as adult records if the delinquent is seventeen years or older.".................