If he has underage nude pictures of you, he can be arrested and charged with possession of child pornography. If he sends these pictures to your family, he can also be charged with distribution of child pornography. Since you sent him the pictures in the first place, you also can face trouble. For example, if a 16-year-old girl sends nude images of herself to boy she likes, she could be found guilty of distributing child pornography. I suggest you contact your ex and tell him to delete these pictures immediately. This is not something either of you want to get caught up in.
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Under federal law, and maybe Cal. law as well, inducing a minor to make explicit photos and distributing the photos are federal crimes relating to child pornography. The fact that he is also a minor doesn't matter. These can be serious crimes. Under federal law this relates to anyone under 18. So you can tell him that he has committed a federal crime and may commit a new one if he carries out his threat. The evidence will be all over his computer which would be subject to seizure by search warrant. Then, if you choose, you can report him to the police or FBI. He may decide he is better off deleting your photos.
My colleagues give good analysis. Further, if any demand for money has been made, make sure you don't delete it since that would be a separate crime.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
You might want to consider a restraining order as well. You could obtain a restraining order in as little as three days and most local courts offer clinics on how to file a restraining order and what type you would need and you would not have to hire an attorney although that is always an option. If granted a restraining order would not ply prohibit your ex from having any contact from any of your family or friends but could possibly extend to "restrain" him from disseminating any information and/or pictures of you.