I'm sure he'd consider it unlucky considering his son is a mothercon. But the king does say 'every few decades' that's a small enough time period for literally EVERY king to have at least one child with that trait. Its entirely possible that its more unlikely to NOT have at least one child who's like that, and could be a cause for their family having so few children to reduce those odds. After all they have 2 sons and no daughters seemingly, they might have been worried about having a second son with the trait and turning their kingdom into super game of thrones if the two get interested in political battles against each other.