What to do if my son was arrested after walking out of a store with a friend who shoplifted without his knowledge but once back inside my son offered to pay for the items?

His first court appearance will probably be very procedural in nature.  The judge will want to know how he want to plea, does he have an attorney, and is he aware of the charges against him.  The judge may allow you and your son to talk to the prosecutor, but be aware that anything he tells the prosecutor can be used against him later.  After the first or second setting, the judge could set the case for trial if your son has not made a decision on whether or not to accept a plea bargain.

The burden of proof at any stage in the criminal process is on the state-- so technically, your son doesn't have to do anything.  However, it never hurts to be proactive in trying to get ready for the first announcement or a final trial.  If you can afford an attorney, try to get one.  If he cannot afford one, ask the court for a court-appointed attorney.  Even though this is a misdemeanor charge, it is still a blemish on his record.  It could affect employment and scholarship opportunities-- which no young man his age can afford to lose.  An attorney will help him present his defense.  He could also help prepare an affidavit for the other friend to sign, acknowledging your son's innocence.  Even if the prosecutor does not believe your son, the attorney could still try to bargain for a divert program or conditional dismissal in exchange for community service or some other condition.  Your son can try to represent himself, but it really is better to have an attorney who is familiar with the procedures and programs that will help your son's situation.