Can my son be charged for simple possession if I didn't know my passenger had the drugs on her?

My son was hanging with his friend, and his friends girlfriend, and they started to drink . My son wasn't drinking so he took the keys to his friends car, and was going to drive his friend home. They were pulled over by a police officer, and due to my son's friends and his girlfriend acting intoxicated the police officer searched the car. The girlfriend had 12 Xanax pills in her purse, and my son was cited for simple possession of Xanax, no proof of insurance, and expired tags. (My son did not know his friends tags were expired, and that he didn't have proof of insurance. I'm honestly not sure if his friend has car insurance. My son is scheduled to go to court, but his friend and his friends girlfriend wasn't charged with anything. I am not sure how this could be. My son was doing the good thing and now he's the one in trouble. I'm very confused about this. Do I need to seek an attorney?