Two points I’ll make in regard to the OP’s question:
1) R’ Mattisyahu Solomon, in his book on chinuch, says that many times, chinuch is simply about having common sense. So use your common sense. If the kid will clearly not respond well to “blunting his teeth”, don’t do it.
2) As bad as this Rasha in the Hagaddah might be, he’s still at the seder. And he’s asking you a question. Which means, you can still engage him in firm debate. If he doesn’t care enough to ask you a question, or he’s not even at your seder, the firm approach won’t work. That’s where you just need to show love.