I agree I’d separate out the racism from the right wing views (I understand on the far right the two can get a bit blurred), and focus on tackling the racism I think for now. As others have said it’s not uncommon for people to vote Tory much as I might personally disagree with it! The black people are not British thing is the only example you have given which I would consider to be out and out racism, and the other things he mentions I think have some arguments on both sides really (although it’s nuanced, I hadn’t heard the thing about the Coco Pops monkey and just googled it, initially I also thought it was a ridiculous fuss about nothing - but then when it’s pointed out that Kellogg was a racist and eugenicist himself who funded organisations trying to prove white superiority then you can understand why people may be reading things in to the monkey on the cereal box which may or may not have been intended).
It might be more effective rather than flatly telling him his views are wrong, to engage in trying to identify both sides of an argument with him (where there are realistically two sides, I think saying black people aren’t British is a non-starter) eg. looking at the pros of immigration and what immigrants to this country contribute etc versus acknowledging that Britain has had a high level of immigration for some time now and that it has changed the social and cultural make-up of many places.
I was going to ask if he had been bullied but then I read that he has. It’s not an excuse but if your experience of white people (family and primary school friends maybe) is largely positive and you then encounter bullies at a majority black school (who are therefore presumably black themselves), I could see how that might be part of forming an overall negative impression of black people. (As an aside why on earth did they not punish the girl?! I think he’s right to say that was sexist of the school.
I think very few white people in the UK have real lived experience of being in a majority black environment for any length of time and I have found it surprisingly difficult/stressful, for reasons I would struggle to explain even to myself (my ex was black and I at one stage went to a church with him where there were only one or two other white faces). Just realising that you are the minority now in terms of not only how you look but your cultural background is surprisingly confronting. It really gives you an appreciation for how difficult it must be for other races being in a majority white environment all the time. So I suspect his essentially being a minority in his everyday life may have a larger role than you think in making him feel “oppressed”. And bullying has probably magnified this. However having black friends should mitigate against this. Another school environment might make a difference, especially if you say you feel the school aren’t handling this well (I agree it’s useful to make sure the school know his views are not coming from the family unit).
I’d definitely have a look at what hes watching online, my son is 13 and definitely holds some views that I was struggling to work out precisely where they came from (eg he is strongly pro trans - I’m not anti trans but it’s just not something we’ve ever really spoken about until recently, and he goes to a school with majority first generation immigrants who for the most part aren’t exactly pro LGBTQ in day to day school life looking at the gay slurs on the school Whatsapp groups). Then I had a look at what he’s been watching on YouTube and realised he’s gone down a rabbit hole of watching videos of the sort of gaming stuff etc he likes which happen to be made by trans people and listened to a few and understood where he’s getting his opinions!
I don’t know I would handle this OP, it does sound difficult. I can understand why people are suggesting Prevent but I’m not sure if this might be overkill, depending just how extreme his views are. I guess I do have some sympathy for young white men right now in that both their sex and race in the UK are coming under heavy criticism at the moment at a time when they are forming identities and unfortunately I suspect he’s not the only one to feel aggrieved about it. To be honest I personally know several white men (no longer teens) who feel similarly “oppressed” at the moment although I don’t totally agree with all their views. Having said that I think we needed both metoo and BLM etc. I just hope we aren’t storing up more problems for the future for ourselves with a massive backlash.