This is a good example of a strong self-esteem.
Self-esteem is the proper evaluation of our value and abilities, involves perceptions, thoughts about ourselves and our way of being, as well as valuations of our character and our body.
Generally, these assessments are made by comparing ourselves with other people or with a projection of how we would like to be. If the assessment is positive, it is considered that someone has a strong self-esteem, on the contrary, if the evaluation is negative, it is considered that the self-esteem is low.
In this case, William has a strong self-esteem, because he knows his skills and considers that he can not work (building with blocks) despite his father's doubts.
I hope this answer can help you.
