This sign of relationship problems is usually caused by a combination of low self-esteem and poor social skills. Children who have good social skills and high self-esteem can handle the give-and-take. Usually, they either do not respond to the taunting or have developed ways of stopping it. Often this means using come-backs at least as nasty as the original taunt. You and other adults may object; but it works.
Telling your child to ignore teasing and put-downs is good advice; but it only works up to a point and part of the time. Also, this does nothing to stop the assault on your child's self- esteem. It is normally better for you to stay out of it as much as you can. While you are staying on the sideline, think about whether your child may be using this problem as a new way to get your attention. He needs to learn to deal with his problem by himself.
A little coaching from you may help. He only needs to learn one or two social responses and use them calmly while looking the taunter straight in the eye. Whatever the taunter says next, he needs only to hold his ground, keep staring, and keep his mouth closed. If your child feels a need to say something, he might try, "Thank you for sharing that with me." If the taunter keeps it up, the social puzzle is just that much more challenging. He has to use the response every time for at least ten or fifteen tries, though. By that time, he has started to be socially pro- active about his problems.
The strategy suggested assumes your child is having his difficulties within a social context including other youngsters who are not exhibiting more serious behavior problems. This means your first focus needs to be on with whom your child is having problems. You might first talk with school officials or others who have first-hand knowledge about the young people involved. If the youngsters involved are likely to exhibit abnormally extreme reactions to your child, help your child understand the taunters have serious behavior problems and he should do whatever he can to avoid them. If that is not possible, talk with the people at school about how to advise your child about handling the taunters' behavior.