A preliminary hearing is a hearing by a judge to determine whether a person charged with a crime should be held for trial. At the hearing, the prosecutors are required to produce sufficient evidence to establish that there is probable cause to believe (1) that a crime has been committed, and (2) that the defendant (your son) committed it.
It is basically a first screening of the charge. Its function is not to try the defendant. Nor does it require the same degree of proof as is necessary for an indictment or for conviction after trial. Its function is to determine whether there is sufficient evidence to hold an accused for trial.
Since the death occurred while your son and friend were drinking/wrestling the preliminary hearing will be to determine whether there is sufficient evidence to say that the death was a homicide or whether it was just an accident.
Your son does have the right to be represented by counsel at the hearing, and I recommend that he does secure representation by a criminal defense attorney.
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