Regardless of the form violence takes, it's better to prevent it than to deal with it after the fact. A parent knows if her child is quick to lose her cool, and can watch closely when her frustration is growing.
Intercepting a bite, accompanying the action with appropriate words and deeds, is important. Especially in the case of biting, when the bitten child's horrified reaction is the biter's reward. We repeat what we find rewarding.
I'll never forget the season, years ago, when one 20-month-old granddaughter would, whenever the temptation arose, gleefully chomp down on the plump pink cheek of another 20-month-old granddaughter. The bitten child's big blue eyes would fill with tears, and she would stand there looking deeply hurt — not about the bite, I think, but about this betrayal by her favorite cousin. For eight weeks or so, we had to watch the pair like hawks. If the biter's black eyes glistened, a wide smile spread across her face, and she lunged toward her cousin, one of us had to leap in. Separately and on-site, we taught the stunned victim to prevent bites by stretching out her arms to block the oncoming mouth, and shouting, "No!" It took perseverance, but over time this strategy was effective. (By the way, these girls are high-school seniors now and incredibly lovely people!)
I understand your reaction and your son's: Biting upsets the child who is bitten and his parents more than any other form of childhood aggression, including hitting, grabbing, and scratching.
Being confronted with the distress and anger parents display when their child is bitten (especially over and over) is probably a big part of why parents are so anxious about this particular behavior. I suggest this cluster of responses:
* Since your child is regularly being bitten by a playmate, you are certainly entitled to tell the biter's parents that for the time being, until they get the behavior to stop, you can't let the kids play together, "because it's my responsibility to protect my child from getting hurt."
* If the biting occurs at school or childcare, tell your child's teacher that you would like to have a meeting with all parents involved. Discuss all of the above with them and listen to their feelings. Whether your child is the person who bites or the target, establish the fact that none of you think biting is OK; that, as a team, you will stop the behavior.
If the problem also exists beyond the classroom walls, the child's parents should talk with the parents of all their child's playmates.
* Reassure the biter's parents that you aren't angry with them, and share some of these ideas so they can help their child. Most likely, they feel bad and don't know what to do.
* Urge everyone to ensure that an adult shadows the biting child to physically block any attempted aggression.
* Think about ways to encourage your child to be more assertive. Empower him to say, "No! You're not allowed to hurt me!"; to move away; to say, "I won't play with you until you are nice to me."
* Give children who bite many ways to feel powerful other than by seeing the look of shock on a victim's face.
* Ask your child's teacher to initiate a discussion during group time of whether or not children like others to hurt them. Each child should have her say. Children who have been bitten should be invited to say their piece. It doesn't help for adults to moralize, but all relevant adults, including grandparents and babysitters, can encourage children to help anybody who bites to learn better ways to act.
* A helpful 4-year-old once explained to me that his dog was a biter. The family bought a rubber bone and put it in the dog's mouth whenever he wanted to bite. The boy suggested also doing this for his classmate.