No a failure of witness identification, by itself, does not require that the charges be dropped, because there is other types of evidence than witness identification. Being with people who were committing a crime, for example, may be evidence of complicity; or having other defendants state that your son was involved in the crime; or any statements by him that may have incriminated him; etc. Ultimately, he cannot be convicted of a crime unless there is evidence beyond a reasonable doubt that he committed a criminal act; however, the amount or quality of evidence required to charge someone is much lower than the evidence required to convict, so there may be enough that the charges do not need to be dropped at this time.