There's nothing for the boy to "figure out". It's the adults' responsibility to teach children; there'd be no civilization if subsequent generations were stuck figuring things out for themselves. He's a boy. He should be taught he's a boy because he's a boy.

What other criteria exist?

"Do you feel more like a boy or a girl" is a nonsensical question. Preferences in clothing can be similarly taught and there's nothing bad about teaching them. It's the parents' responsibility that the child grows best adjusted to adult life, which sometimes means tempering expectations. Presenting yourself is an important life skill. He's eventually going to look stupid and ugly in a dress and thus drastically decrease his chances at a happy life.