He lacks an independent identity.

"I exist only in relation to others."

Pay attention to him when he is not in competition with others.

Remind him that he is who he is without comparing to others.

Give him some age-appropriate responsibilities. Praise him for a job well done.

When in any situation, ask him: where is <his name> now? Do you feel your existence?

Next time he loses a game, say: you are not the game, you're bigger than the game, control your feeling.

Reward him when you observe his attempt to control himself.

He needs to establish that "self" in his mind.