This is the most difficult part. He has to stop believing the voices are real before much improvement will occur. I had so many arguments with my cousins about this. It took me quite awhile to fully accept it. I think it helped that people my own age who I loved and trusted were telling me that the voices were not real, that they were just being created by my own brain.
And yes, sometimes (really for me, most of the time) the voices can be quite amusing and entertaining. I was witty, sarcastic, and snarky before my first psychotic break, and the voices reflect that because they are being created by me. It is easy to get sucked in by them and begin to like them when they say outrageously funny things, and sometimes they say things that are so absurd given the situation that you almost can’t help but laugh at them. And then it just feeds the delusional thoughts and makes them grow more believable and more powerful. I have learned to not give into laughing at them out in public, or around anyone else, but sometimes when I’m alone in my room I do laugh. Which is one of the reasons it’s not good for me to spend much time in my bedroom.