Yes, very much what HK said.
If he's never been one for tantrums about losing other games, but is doing it at chess, I would suspect it's because a bigger deal has been made (maybe not to his face, but kids hear stuff) about how GOOD he is at chess. Other games are kid games, not a big deal. Chess, he's in grown-up territory. And as a perfectionist, I can tell you that if you're known to be GOOD at something, and you lose, that is incontrovertible proof that you are clearly stupid and incompetent, and it's time for a tantrum.
