Willie is quite simply the victim of domestic abuse. It becomes apparent later in the novel that his mother is mentally unbalanced as seen through her cruel discipline of Willie based on extremely austere Christian beliefs. Willie is taught to believe that his a very bad little boy and subsequently needs to be punished regularly - especially through physical beatings.
When he comes to Mr Tom he is a little boy who has no concept of being valued and loved. He carries all the associated symptoms of child abuse being a great reluctance to communicate, great fear of seeing an angry reaction in people and bed wetting. He owns virtually nothing (including adequate clothing) and has had virtually no experience in every day activities like visiting shops or even going to school. Willie first comes to live with Mr Tom in a highly damaged physical and mental state.
willie thinks that he is a wate because that his moyher told him like that so many times and whenever his mother beats him.