Just a thought, maybe a stupid one, but....
A lot of voice hearers use a cellphone to talk to the voices. I know I do. It helps to "cover" the conversation, kind of beats talking to the air. I personally have an old non working cell I use in pubic places so as not to look too crazy. One thing I have learned is that when people try to push through hallucinations and tell me they're not real, I get defensive and it often pushes me further away. They ARE real to me. Maybe you can acknowledge that you know he's hearing these things, and let him know he can tell you about them. Try to keep him in real conversations as often as possible, until he gets to the point where he wants help. (of course if he's violent or in danger you need to get help asap)