Unfortunately this sort of interaction among teenagers is pretty common, as they tend to have poor impulse control, be prone to risk taking and have high libidos!
Your son is far from alone. Please reassure him that, although technically he has broken the law (distribution of child pornography) he is very unlikely to be prosecuted, as he has engaged in the cincensual sharing of nudes with someone he believes is the same age. If the police tried to prosecute every young person who does this they would literally be doing nothing else.
Some important questions:
A) has he ever met this girl? Or facetimed with her? Does he have any evidence she is who she purports to be?
B) did the video he sent include his face?
He will be likely feeling very upset, scared and even violated at this point. It's good that he feels he can speak to you about what has happened - very many children are scared to be open with their parents about these things and that does SO much harm - so well done to you for fostering the kind of relationship where he knows he can turn to you for help.
He may need further help in the future, maybe counselling, depending how distressed he is by this experience.
If the video does not include identifying details, I would suggest the best course of action is for him to block the "girl" on all platforms and try to move on from this situation.
If I does, and if you have reason to believe she may not, in fact, be the teenage girl she purports to me, I would suggest reporting this, if your son is comfortable with that.
If you turn to his school then safeguarding officer should be able to advise you further. Really, you need to talk to someone about this face to face. It's hard for us to advise on here because there are so many variables we would need the answers to first.
There is help out there though. Sadly it's a really common scenario.
Good luck.