A good quote to inspire a nephew is from Theodore Roosevelt, who said, "The only man who never makes a mistake is the man who never does anything." Depending on the nephew's age, he may need help in different areas, but it is always crucial for him to understand that trying and failing is a natural part of life. A reminder that the only way to succeed is to try and providing him with a quote like this can uplift him without pointing any blame.
Young nephews who are still growing have to deal with a lot of stress in school, and some amount of failure, frustration and confusion is inevitable. More boys than girls drop out of school per year. Keep your young nephew on the right track with quotes to help him identify his confusions so that he do not feel alone in his struggles as he grows up. The key to supporting a young person like a nephew is not avoid being pushy and also to recognize his successes first. In accordance with this quote, try appreciating your nephew for the fact that he tried in the first place before recognizing that he may have failed in an endeavor.