I assume these are digital photos that were electronically transferred (not prints physically delivered). If they were prints physically delivered, he owns those prints, since you used to own them but you unconditionally transferred ownership to him by giving them. No backsies under the law.
The photos are protected by copyright law, which means that the person who took the pictures has the exclusive right to make copies, disseminate them, and authorize making copies. In order for anyone to make a copy, they need permission – a license – from you. In the world of pre-planned business deals, the copyright holder writes up a document granting B some right to use the protected material, which typically means "you can install it on your various devices but may not give copies to others". In this case, however, you didn't create an explicit written license. So if this ends up in court, the question is what implicit license you granted. The courts will not decide that you granted him the license to unrestrictedly sell or give away copies of the protected material. The most likely outcome would be that he can only keep his copy, i.e. he will not be forced to erase the copy that you sent him. What the courts would do is try to discern what license you most likely intended to grant to him. There is a provision in copyright law that allows a licensee to make backup copies of a computer program (17 USC 117), but a digital photo is not a computer program. So the lifespan of the copy that you sent would be the lifespan of the phone (I assume) that you sent it to.
Since actually using a digital photo technically requires making a copy (from disk storage to computational memory), there is a legal direction (dead-end) that you could go where the photo could exist on the phone, but never be opened again. Again, the courts would have to discern what license you probably intended w.r.t. ever opening the photo – obviously you intended that the file could be opened / viewed any number of times.
You could argue that the license which you granted was conditional, i.e. "you can have and use these pictures as long as we are a thing", but establishing that this was part of the license would be tricky. Free digital content often has some "as long as" condition attached to it, i.e. "you can use this program as long as you are affiliated with University of Whatever". I don't consider a conditional license to be a ridiculous interpretation, on the other hand the particular court (judge) might decide that people who sext should be forced to live with the unpleasant consequences of their decisions. If we exclude such a line of thinking, I don't see a compelling counter-argument that your ex-partner inequitably loses a right by construing the license as conditional. I don't know if there is any case law that addresses this: at any rate, copyright law would severely limit what he could do with the pictures (the tort "invasion of privacy" also limits dissemination).
