What a stupid thing to say. If he's bored the teacher can advance his grade faster or at least give him personally more challenging material.
Should we never teach anyone anything, just to avoid they'll be bored when they see something they already know? Is starting to learn at 6 objectively "better" than at 2? No, that's just how our system currently works. And what you're saying is you want your kid to be mediocre. Great, but the rest of us are aiming higher.