E.S.
You would love Canadian child psych Gordon Neufeld's book: Hold Onto Your Kids: Why Parents Should Matter More Than Peers.
It is a strong reminder that the most solid and true base our kids will ever have (emotionally psychologically and socially...is us, the parent/primary caregiver. It is what will allow our kids to blossom into real emotional maturity. Peers can never help with that. Peers are emotionally weak...they bully (or side with bullies) and they crush individuality, even when not bullying.
Bullies need intervention...they are sad, hollow kids who really have no friends...just people who fear them. A bully will never have a full life...always blaming others for his/her actions.
Make sure your son knows you are always there for him...unconditionally. Good grades, bad grades. Good behavior, bad behavior. Good day, bad day. He needs to know that although it is fun to have friends, often they come and go throughout our lives. It is better to have no friends than ones you fear. You and other trusted adults are his true vacation from the petty garbage peers dish out.
Home schooled kids have an advantage over kids in any school...big or small. They do not have to navigate the dangerous waters of UNSUPERVISED peer interaction and relationships ALL DAY. It allows for more time to really study. This is also why uniforms and separate boys and girls math classes are making a comeback in public schools.
Telling kids to be nice to each other just does not work. If you leave your son in this school you are going to have to go into the adult leadership there, and make your case for things to be taken seriously. Also I'd make it clear to your son that he does does not have to be nice to everyone. Civil..yes. Nice...NO.