This is the underlying problem causing your child's low self-esteem. He feels like he does not fit in, does not belong. This goes against what may be the most human of human needs: the need to belong. Children are who others tell them they are; and youngsters with very low self-esteem have heard their world telling them they do not belong anywhere. They think they do not fit in, do not belong. As sad as it is, they are partly right. Whether it is at home or somewhere else, to some extent they do not fit in or belong in what is to them an important part of their world.
This makes helping your child with his self-esteem problems doubly difficult. Help him with his ideas about himself that are not true as well as with those that are true.
For example, your child says, "I don't belong anywhere. No one cares about me." Say, "You belong here and I care about you; but you think that is not true. I don't understand why you feel that way. That doesn't help you much right now. I feel badly for you but am not going to tell you how to feel or what to think. I'm here for you and hope that helps at least a little. I hope you will hang in there with me and keep trying to help me understand why you don't feel like you belong and why you don't think I care." The idea is to actively value and respect your child enough to deal with the truth, with how he really feels.