D.B.
At 10, we have to start teaching our kids resilience, and about the reality that some friendships run their course. What your son is feeling as rejection needs to be flipped around so he sees that he is standing on principle and for values (e.g. defending a 6 year old). It also helps to know what the "bad words" were and whether it was okay for the 6 year old to hear them. We have to teach kids that someone saying "You're a bad friend" doesn't mean that you actually are one. And we have to teach kids that if someone shows you who they are, believe them. Now, that has to be a little flexible with children who are still learning how to navigate relationships. So we sometimes have to be forgiving and allow for personal growth in our classmates. We also have to teach our kids that it's possible to be in the same school or classroom with people who aren't our best friends, and also that's it's possible (and often desirable) to have a range of friends with different interests and not always have a "best" friend. It's a good time to teach about exclusion. While your son feels he's the only one by himself, my guess is that he is not looking around very carefully to see who else is sitting alone or in need of friendship. Out of 100 kids, there cannot be 99 who get along and just your son who is alone. Teach him to BE a friend, not just to select certain kids or choose certain activities to be involved in. It's fine for him to have unique interests, and to start to see the strengths of that. But he has to be open to other kids and not decide, "Oh, that one isn't fun" or "that one doesn't like what I like." That's using the same snap judgments that are so hurtful to him when he receives them.
I'd get some books out of the library about unique "characters" in history, people who were, perhaps, introverts or those who struggled socially, or who learned certain things later than others, and who went on to excel in some unique area. And teach him that we don't run away from every problem, slight, or thoughtless remark. There are kids in every school who are jerks or who are mean, or who are in need of a friend. Tell him, in simple terms, that we all have to learn to work with people we might not like that much, and we have to deal with teachers (coaches, scout leaders, even relatives) who have different styles and personalities. We don't run away. We learn to cope. If your son's hurt feelings are causing him to mope around or be isolated, he's not going to be as "attractive" as a potential friend. Some schools have a "buddy bench" but your son can be taught to look around at who is sitting alone at lunch or who doesn't have a partner for some class activity, and he can make the overtures.
If you feel this school is too small and not a good fit for him, that's another story. But that has to be based on a whole lot of criteria and not just on something one or two kids said in the 2nd week of school. I know it's a huge deal to him, and I know it's just awful to see your child hurting, but this is a "teachable moment" of how to negotiate friendships and how to ride out the bumpy road of social relationships.
Parents sometimes want to step in and talk to the other kids or to their parents to smooth things over. Please resist this urge - it hardly ever works out, and it doesn't teach our children the skills they need to develop a thicker skin and to be less afraid to show their interests that will "attract" others.