OP I came on here a while back to ask about the boarding houses for Sherborne for one of my DS.
I don't know about the bullying. Before we applied I asked some friends and a relative who had or have DC at the school, and this issue was never bought up. Considering DS is soft spoken and academic, I would have thought I would have been pre-warned, and I did directly ask as he is at a day school currently, so it is a concern.
When I asked my question here on MN, I did get DM replies and I have now mentioned your post, hopefully, one of the parents who currently have DC there can give you direct information.
You still have plenty of time if your DS is still only 8, to go back again and again, especially to speak to the Housemasters. Everyone I spoke to said the Headmaster is very encouraging about being kind and would not tolerate bullying behaviour. As a side note, I just wanted to say that Sherborne is one of the very few old top traditional schools to publicly show support for BLM - just something I noted as I saw it as a sign of empathy.
Have you thought of getting your DS to Pilgrims for y6? since they are probably the biggest feeder to Winchester. And I guess it would help if your DS is a chorister too - I don't think its too late to get into various choir programmes at his age.
Good luck.