One place to start would be with looking at social stories. Google losing and social story and you will pull up a few links. Here's one: http://www.boardmakershare.com/64624/Winning-and-Losing-Social-Story

And, just by way of encouragement this was a huge issue here at that age and eventually it faded. Learning some rote phrases "good game" etc. made a big difference. I would suggest giving information and then opportunities to practice how to react again and again.