How do I build confidence in a young athlete?

Here's the scenario. This kid is awesome, top of the game. He puts in countless hours practicing, working out, and getting private lessons. Problem is, come game time he is a nervous wreck. Often it means he starts to make mistakes, gets flustered, and can't get his head straight. I've tried all sorts of positive reinforcement. We are at a tournament right now where the best in the world are scouted- getting in is by invitation only. Today he totally floundered and blew the game on an error that shouldn't have happened; totally his fault no doubt about it. Other Adults have been saying it's a team effort, he played great, and besides they are still kids and make mistakes. But now he is all freaked out and it's not over. He has played with n ability that makes the crowd gasp, but then next game he is a head case. How can I get him to realize his full potential? He wants and sees a future in this, and that is quite possible if he can pull it together. What can I say or do?