I haven't had any experiences in this area so keep that in mind.
Are you sure he is actually hearing voices or do you think it could be some kind of exercise of imagination? If your family is religious; Is your family church very focused on the evils of the devil? It is possible he is having social issues with the kids in school and then hypothesizing they are in league with the devil, if it this sort of thing is a frequent motif of your family church's liturgy. I am probably not expressing this right but if he hates the kids at school because they bully him, he might imagine they are doing something like that in order to explain in his own mind why they are such bad people.
If that is the case, and he is not actually hearing voices, then maybe you can get him to tell you about the kids in school and what they are actually doing (bullying or whatever) and address it on that front, while also letting him know that being a bully does not mean someone is in league with the devil.