I think you discuss potential responses to bullying with him so he has some idea about things he could say to stand up for himself but not have to fight. And make sure that he knows he can (and should!) always come to you if there's any sort of problem at school.
I'm not sure I'd worry too much about bullying until it happens, though. Sometimes it happens and sometimes it doesn't. Just being younger doesn't automatically make a kid a target for bullying. That fear alone certainly wouldn't keep me from taking advantage of a good educational situation, personally.
Prep him as best you can, keep your eye on things, maybe even let the teacher know of your fears so she will watch out for problems. But there's no reason to let fear stand in your way here, I don't think. After all, it may not ever even become a problem.
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Here's hoping!
And to back Dottie, who snuck in there on me, shoulder-shrugging is one very good response to potential bullies. It's hard to have fun attacking a kid who just doesn't get worked up about it.