Illustrate the Consequences
Kids can't always understand the consequences of dishonesty. Instead, it's a knee-jerk reaction to prevent unpleasantness, so it's gratifying in the moment. It's important that your child really understands where lying gets him. Once he comes clean, lead him through the chain reaction that occurs with the lie. For example, explain that lying about grades means you can't help him out. If he tells you when he's struggling, you could hire a tutor or speak with his teacher about extra credit, but as long as he conceals the truth, he's on his own. This should help him understand that lying is more than just a one-time thing.