Low self-esteem starts with worrying and fretting about failing. It grows into giving up quickly. This leads to shutting down and not trying. If parental or other adult reactions are too harsh, your child goes through the motions for fear of even harsher consequences. He simply plays the game. This is a very sad way for him to think and feel about success, achievement, and interpersonal participation. Nonetheless, low self-esteem can get still worse for him.
Your youngster feels incompetent and may have a low physical-sexual self-image as well. Most children have some uncertainty about themselves physically and sexually; but children with self-esteem problems have these thoughts persistently and feel them strongly. They think they cannot succeed, cannot achieve, and they are not accepted socially. They also may think they are not made right or well enough.
This poor self-image has nothing to do with how the young person actually looks or is developing. Your child believes it, no matter what the facts are or what other people tell him.
Resist trying to convince him his perceptions and feelings are wrong. That only strengthens his belief you do not understand. You can say, "I feel badly you think about yourself in such negative ways. You're a normal person and are fine physically and sexually. I know you doubt that even though it is true. I want to hear what you think and feel about it; but I'm not going to argue. You have a right to your feelings. Will you tell me how it feels to you?"