"You wouldn't." Because he is a kid, and to him it would be an adventure. Of course he'd want to be a superhero--who wouldn't? But it's an adult's job to say no. Not encourage him.

"I'm aware in your world, for whatever reason, this shit makes sense." The reason being that his world is fictional wish fulfillment is probably not a great one to bring up. Presumably in universe there are reasons that are better than that. "But if it happened in our world, even putting aside the physical improbability that someone could take that much damage that often and still be walking, he'd be extremely arrested. You don't get to just declare yourself the arbiter of right and wrong and get away with it. The systems in place aren't perfect, but they're still more just than guy dressed up as a bat."

Then again, rich white guy. He'd be fine. Maybe Batman's not so unbelievable after all.