No, it is not likely the charges will simply be dropped. Being mentally unstable when you commit a crime usually results in an alternative sentence, such as committment to an institution or mandatory treatment, not dropping the charges entirely. Speak to a criminal defense attorney before doing anything: he may be better off with the punishment for these offenses than risking institutionalization, which could occur if the court ends up convinced he can't control himself and is a threat to others or himself (unless, that is, you believe that he should be institutionalized for his own good).