My daughter has faced similar situations though mostly with gender overtones rather than racial. She deals with them pretty successfully using her big sister skills. She has described it to me like this. I called up my old crone persona, looked him in the eyes, projecting disappointment with his behavior and sadness that he felt he needed to protect himself that way, then kindly said “Next time you want my help, just ask me.” Then I treated him as if he were the person I thought he could be and gave positive reinforcement. That usually does the trick for her, in a school environment.