Can they change charges after he was bailed out?

My adult son had a misdemeanor warrant and an officer claims he recognized his vehicle so he detained him. While searching him, they found a very small amount of meth residue in his pocket so he was taken to jail and charged with suspicion of possession of meth. He was in jail for 3 days and was taken to private room and 2 detectives were insisting he work for them doing controlled buys. He made it clear he was not interested in doing any such thing to both detectives. I then bailed him out. On his paperwork and his court papers at time of bail, he was arrested and his charge was for suspicion of possession. When he went to court date 2 weeks later, his charges were now possession. So due to the fact he refused to work for them they changed his charges. I have been researching the state laws but I am unable to find anything to reference this case to in library. Does anyone know where to research this type of situation?