"I try to create a mental image in my brain where there is a glass wall and everything that i don't like gets pushed up against the glass wall, it breaks and turns into sparks like fireworks." That is what works for my OCD. Like sometimes psychotic says, mental visions could work for voices as well i never tried.

Here's a good site that your son can visit: Intervoice | Essential Facts about Hearing Voices

Also, sometimes the first, second antipsychotic might not work so you should tell him that so he has more hope in this situation. Most times it's just a matter of increasing the dosage although I could be wrong.