For a 9yo and that sort of age, when my girls were young I told them this and maybe you could go with similar.
It might happen that another kid at school shows you something like a pic or a bit of a video that has naked grown ups in it. If another kid or anyone shows you anything like that then please tell me because it's not for children at all even older children.
If you see anything like that then it could be that they're doing scary or strange things. It's something some grown ups are interested in but not all of them. And it's not real, it's made for the people watching, it's like a film, nothing to do with real life.
If they have an idea what sex is then.
It's totally not how mummies and daddies (or whatever) behave in real life. It's a made up fantasy.
(I have two girls and said made up usually for men to watch but you can skip that!).
And he can and should talk to you about it if he is confused or wants to.
And well done for taking you, he did the right thing.
And
Find out who the boy is. At that age. Tell school? I don't really know about that side.
But if your son knows if he saw a clip inadvertently or saw a lot or the whole thing. Sounds like maybe a lot.
Incest? Bloody hell. Adults not a child I hope?
If he saw loads. If it had a child. Then school and I guess police? Again. Not sure but need to get it looked into.
If adults what was relationship? And sounds rapey?
I'd say it's highly illegal to do anything like that with your family and it means prison.
That in real life its not a thing that happens hardly ever.
It's made up not real at all. You don't know why they'd make that up as it's horrible but nevertheless it's all pretend.
How rough?
I mean don't press him or try to find out from him it will make things worse. but do you have as idea already.