It may be normal behavior, but I'm sure it's against the school rules! How does your DS feel about reporting the incident? Is he afraid of the bully now?
I would approach this with the school as a bullying issue. The bully was also picking on another kid (who presumably wasn't accelerated...) I don't think they can argue that your DS is too young/small/socially immature/whatever and therefore somehow deserved or provoked the bullying.
Focus on the bully's behavior. What the bully did was against the rules.
