Today we will address the effect of bullying on self-esteem.
Self-Esteem: The effect on a child's self-esteem is the most concerning part of bullying (typically any physical scars heal much quicker than the emotional). It is important for parents/adults to try to protect and rebuild the self-esteem of a victim of bullying. The effect on self-esteem can be varied and seen in different ways. For my nephew it was evidenced by him asking his mom to go back on his ADHD medication (he was taken off of it because it was determined that he didn't actually have ADHD) because it suppressed his appetite and made him lose weight (his weight was a target of the teasing).
Helping a child maintain and rebuild self-esteem can be difficult because like most people, they pay more attention to the negative messages they receive. However the more positive messages they receive, the more they will internalize those messages, and eventually this will make it easier for them to dismiss the negative.
So praise, praise, praise your child. In addition, highlight their strengths. Encourage them to hang out with positive peers, who share common interests with them. Encourage them to participate in activities that will boost there overall self confidence.
With that in mind, make sure you are encouraging activities they are interested in. Even the most well intentioned of parent can do more harm by encouraging their child to engage in an activity that they don't like or aren't good at. So even if you think your son would be more popular and have a good time playing football, don't encourage/pressure him to do so when he is bad at sports or just plain doesn't like them. In many cases this may be part of the reason he is getting teased and by you pressuring him, you are inadvertently sending him the message that his bully is right and there is something wrong with him.
For additional resources on bullying check the U.S. Government's website.