This is called "selective mutism".
You'll want to print out information and give it to his teachers, so they might understand better:
You should certainly look into selective mutism as a possibility. The SMG-CAN is a not-for-profit devoted to helping families with selective mutism www.selectivemutism.org. They will be co-hosting the annual SM Conference with my organization, the Child Mind Institute www.childmind.org on 10/28/2011. Please check out our websites for more information.
We also run a really cool program called Brave Buddies - a weeklong analog class/camp for kids with SM http://www.childmind.org/en/clinics/programs/brave-buddies