Only the DA can press charges, not your son, not the police. If your son told the police he didn't want to proceed maybe they didn't pass it along to the DA. Get a copy of the police report if you don't already have one and sen a copy with explanatory cover letter to the DA.
As for the time limits it depends on whether the charge would be a felony or misdemeanor. The shortest time limit in CA is one year from the incident.
If your son suffered physical or emotional damage as a result of the incident, you can file a civil action for damages against this person.
You can get a copy of the police report for a fee. This will have the defendants name and address for service purposes. If he's admitted the crime in the police report that will go along way in proving your civil case.
Edward J. Blum