Unfortunately this happens to black & POC a lot. Regularly getting compared to something unfavourable, which then can perpertrate further racism.
It is racist, but unfortunately it's a more of a 'hidden' form of racism, that unless you've experienced it you are likely to not understand it.
If your son was the only white person in the class and he was labelled on a picture of say, Jimmy Savile, it would be horrific and racist. It is essentially saying that this is what all people of this colour look like/do and that person is associated with something negative.
OP- maybe you could report the WhatsApp but realistically I'm not sure what the school can do other than have a 'talk'during assembly or PHSE, and emailing the parents about disturbing messages
They have no control over what the children do on social media, so I doubt anything will change.
Yes the sexualised messages are a concern but also could be puberty. I do remember girls asking boys about whether they had big books and saying stuff like giving head- some of these were my friends and had absolutely no history of any fork of exposure or abuse- they did not actually mean it.
The difference now is that all these horrid things kids say, are now on social media where they can be looked at. No excuse but kids say the worst things and really do not see the impact they have.
At the moment at my sons school there's a big problem with having 'The N pass', essentially a black child gives their non-black friend permission to say the n word. Do they realise its awful and racist? Not in the slightest. My son is adamant he does not do this and is aware to never say it himself but who knows?