Get him to work out and get bulky-huge. Strangers comment because he probably looks like a stretched out kid.

No.

This is bad advice. Don't change your physique and biometrics so that other people/strangers have an easier time understanding your existence. If the kid WANTS that then fine, go for it; but don't tell him the secret to happiness is satisfying other people. He should not under any circumstance change who he is so he can fit into other peoples preconceived notions of body proportions. You seem sincere and I likewise would rather this kid be comfortable in his own skin but conforming to other peoples predilections is the wrong way to do it.

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I think it'll get better once this kid develops a sense of self/self-awareness and can identify things that make him happy.

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ALSO FWIW don't most 15 year olds go through a period where everything sucks? It just might seem exacerbated in his case because he feels like he can source it to being uncommonly tall.