I'd mention to him that you noticed this only happens in K and ask him why he thinks this is so.
Follow up questions:
Do you feel angry sometimes outside of K?
If so, what helps you keep your control?
Is it easier to keep your control outside of K than in K?
If so, why do you think that is?
An emotional response scale exercise may help too. Good luck!
http://giftedkids.about.com/od/socialemotionalissues/qt/emotion_coping.htm