When I worked with toddlers a few of our children kept toys in the center that they were allowed to bite when they were frustrated. They used teething toys or stuffed animals (yuck). I also work with children with sensory integration issues who benefit from having a flat finger placed across the space between nose/mouth when they are "mouthy." These are primarily kids who bite or chew anything and everything just for the sensory input, not just aggressive or frustrated biters.
A better alternative for children who can speak is to consistently teach language to deal with frustration. The most frequent times the children would bite were when a child was in their way and wouldn't move, when a child had a toy he/she wanted, when one child took the other's toy, or even if another child was just too close. Good words for your child to learn are "move," "mine," "my turn," "give it back," etc. Set up situations where your child would need to use these words at home, and teach him how to use them. He won't easily learn to use these words if he is only exposed to them when he is extremely upset, so you need to set him up to learn them in a familiar and comfortable environment before expecting the skills to generalize to school and large-group settings. Talk to the teachers about what you're using at home so they can reinforce at school.
Also make sure the behavior isn't reinforced with either negative or positive attention. If he bites, stay calm and don't give him the reinforcement of an extreme emotional response. That tells him that biting is an effective way to get the adult's attention when he is having a problem.
You obviously can't just IGNORE the behavior in this case, however, so just try to stay calm and avoid a major emotional response. Keep your voice low, and tell him, "You bit your friend. That is not ok. You can say MOVE PLEASE," and remove him from the situation without a lot of extra words or drama. Don't give him what he was trying to get or react to an ensuing tantrum -- he needs to learn that even if a friend really did take his toy, biting is not an effective way to achieve anything.
Too bad we can't control the extreme response of the bitten child, but we can only do so much =D
Good luck!
My son had this same problem. He did outgrow it -- but at the time, our only solution was to take him out of a day care center and place him with a private sitter where he did not have to interact with so many other children.
The teacher says it's "unprovoked at times"? Meaning, usually it's provoked?
Children who bite are usually (not always) trying to defend themselves. They bite bigger stronger children because they have no other recourse to defend themselves.
Biting is a very effective way to fend off bullying other attacks - is that what's usually happening to him?