criminal trespass in a mall

My son was arrested and charged with criminal trespass in the state of Georgia. His actual crime was vandalism. He was picking at the grout on a fountain in the local mall. Why didn't the police charge him with vandalism? And should he plead guilty to criminal trespass? He was in the mall during business hours when the mall is open to the public. This is his first offense. Could he actually get jail time? He is 19.

Thanks.