For any criminal charge you need to hire a lawyer. The government will have a lawyer and he will have all the resources of the prosecutor's office. Even though the penalty might be slight for what your son did it's better not to have it on his record at all. Check Avvo for CA licensed juvenile lawyers.
In Vallejo, the courts are far more concerned with juveniles who commit serious felonies than with a kid who is pulls a prank. Throwing eggs from a car is pretty minor when the police are dealing with drive-by shootings.
If your son doesn't have a record, it is very likely the probation department will handle the case informally. He would meet with a probation officer and be ordered to stay out of trouble; if he doesn't screw up for a specified period of time, the charged would be dismissed.
If you don't hire an attorney, the court will appoint a Public Defender to represent your son. I worked for the Solano County Public Defender's office and it's one of the best in the state.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.