How do I encourage my son to be positive about his own body?

I have a 15 yo son. He's always been on the bigger side, but he's still healthy. We teach him all about Intuitive Eating, which means eating based on one's internal needs rather than a diet. That's why he's healthy even if he is chubbier.

The problem is that other kids don't treat him well. Apparently, he's bullied by some of them because of his appearance. We've tried to make him feel better about his body, that there's nothing wrong with it, but it hasn't worked. Now he's trying to go on super strict diets and overexercising. We don't know what to do.
How do I encourage my son to be positive about his own body?
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