I'm a teacher who had a Y8 tutor group (state) last year, and the things some of the boys had seen online (without seeking them out, necessarily) were quite disturbing. They are invited to group chats with older students (usually male) who they know from being on the bus, or from clubs etc outside of school, and then some of these students share photos and gifs from porn, presumably without thinking 12yos in the group will see. It's really worrying (to me at least) and a real problem, I think.
I'm not sure if this would be any better at a private school, it would probably depend on the school. At my previous school we did have a few students move to us when private didn't work out for whatever reason, and in some ways they seemed more mature than their peers in both good and bad ways, it was quite a small private school where they often socialised with older boys.
I'm not saying this is a problem with all boys of this age, but I do think it's increasingly common with large whatsapp group chats etc, unfortunately. And it's very easy to find these images etc on common social media sites like twitter and tumblr. And of course you get influencers like Andrew Tate promoting a really awful attitude towards women.
I think if it were me, I'd visit the private school (if it's a realistic option, include fee increases to cover increased bills etc) and ask if I could meet/chat to students- you may be able to get a sense at to whether your DS would like them.
I know a year is a long time to be lonely for, but if your current school does options in Y9, that can be a time when students find a new group of friends, so it may not be like this for ever.