If you are under age (generally under 18 in most countries):
Nothing. Report him. He is breaking the law. And you will be breaking the law if you respond. If he tries to blackmail you, again, leave the conversation and report it. The law is on your side if you are a minor.
If you are of legal age:
Whatever you want. It is your body. However I personally would advise turning him down and walking away. You need to be respected for who you are, not because of your body. This person is clearly not interested in your personality or forming a lasting relationship with you. he just wants to use you. Again if he tries to blackmail you, report it, the law is on your side.
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If you know that you are a youth person and don't want them to know about your body them you have no need to tell them anything.