If children hear negativity about themselves they tend to react negatively. Rather re-enforce the positive. Be clear in what you want from him. Don t just issue a vague demand of Be good . Rather specify exactly what it is you want him or her to do.
Children have a knack for changing or avoiding the subject.
To do this he may look away or start talking about something else. Draw his attention back to you and repeat whatever it was that you said. Make sure he understands what you are saying. Go down to his level, look him in the eye, if he tries to look away gently take his arms and repeat what you say. Try not to threaten him or plead with him, this will not gain you any respect and once you have lost respect you have lost the war.