It could be but also disorders like bipolar disorder can cause this too. He would have to have several more symptoms before they would diagnose him with schizophrenia anyway.
It would be worthwhile to ask him when this started and if it comes and goes at times or if the voices are constant. Then this could be discussed with a psychiatrist but regardless it would be essential for him to seek help as there are a variety of diagnoses from bipolar with psychotic features to schizoaffective disorder (schizophrenia with a mood disorder such as I have) that have psychosis and mood disruptions and only a psychiatrist would be able to provide a specific diagnosis.
i have schizophrenia and i used to hear voices until i took risperdal :)