@PepperPotato I have a DS currently at Winchester. I can't comment on whether the school would be the right one for your DS, but I can tell something of our experience of the school.
Winchester has a reputation for suiting less extrovert quirky children, and based on our experience I think this is true. I've found that the pupils are respectful to each other and readily accommodate individual differences - whether someone is interesting to have a conversation with seems far more important to the pupils than whether someone is socially adept. DS claims "there are no extroverts at Winchester" - I'm sure this isn't strictly true, but I've certainly see very little rowdy behaviour. It's certainly a gentler environment by far than my older DS's day school, which for all its positives has a very alpha male culture.
I'm sure some bullying does occur because it does in every school, but it hasn't figured on DS's radar at all (either as direct experience or hearsay) - again in contrast to my older child's school. Sweeping generalisations that all boarding school pupils are bullied by other pupils and staff are just as unhelpful as sweeping generalisations that no bullying ever happens...
There seem to be quite a high proportion of ND pupils at Winchester. Of the ones I know personally, some seem to be thriving and others struggle more with communal life but seem happy to remain at the school. Group extracurricular activities are encouraged, but there are plenty of options for those who prefer more solitary pursuits. It seems to be regarded as pretty normal for pupils to prefer their own company at times. Social cliques don't seem to be a big thing - I get the impression from DS that the social mix is more fluid with boys hanging out with different people at different times depending on who is doing something that interests them. Social events with other schools are available but not compulsory, and there is no stigma associated with not attending. Accommodation is in large dorms for at least the first year or two, and after that it varies by house - for example in my DS's house boys are in shared twin rooms by Year 11 and single rooms in sixth form.
It's worth bearing in mind that in addition to the full weekend exeats (Friday lunchtime to Sunday evening) every 3 weeks, pupils can also go home during the day on Sundays, typically from late morning until 9.00pm. Most of the pupils who live close enough to the school do go home on Sundays, especially in the lower years - there is a big exodus by train for those living in London.
Although you need to accept the Winchester place now, unless the procedure has changed drastically since we went through it, you don't need to commit any serious money until the autumn of Year 8. So if your finances allow, one option would be to accept the place now and wait and see how your DS develops over the next year or so before making a final decision. In the meantime, I can recommend attending offer-holder events and Wykeham Day in the summer term to get a better feel for the school and the pupils. Please do feel free to DM me if there is anything else you'd like to know.