What should we do if my son was cited for meth paraphernalia but found no evidence?

The police officer asked my son to empty his pockets, the pen he found and put in his pocket to throw away was tested, traces of meth were found on it. My son told the officer I do not know where that pen came from but that he had lent his car out the night before to a friend whose car was not running. The police officer read my son his rights, handcuffed him and drove him to his car to search it. My son told them they would not find anything and they did not. The officer looked up my sons nose and said his nose was irritated, we have all been sick for the past week. The officer was going to take him to jail but he received a call for a domestic violence situation, he told my son he was going to cite him and dropped him off at home. I feel that the officer had no real evidence that my son was doing what that person said and the officer had no right to even search my son. Where do we stand on this situation? Thank you.