He needs to be more self-aware

 

His subjective sense of the "self" is small and weak. That leaves much of his objective-self exposed and vulnerable.

His objective-self instinctively senses that vulnerability and seeks cover. A win provides that cover for the moment. A loss makes that exposure unbearable. And crying provides a cover the subject-self is either unwilling or too feeble to provide.

Tell him to take responsibility for his part of the outcome, win or lose. He is worth it. He is good enough to take such responsibility.

Help him grow his subjective-self by cultivating his self-awareness. He exists regardless win or lose.

 

(Parents' reaction to his win or loss may also be a factor. It can be subtle.)