My only suggestion would be to be there, where?, THERE as much as you can, until he's calm again. That and if you can afford it, a little bit of therapy I suppose, just to make sure you're not missing anything and that your child has the neccessary skills for standing up for himself.
I'm sure the other ladies will have loads to add.
I'm so sorry your child has to breath the same air as such an awful child! I also worry about how to handle this sort of thing. Please let us know of all success you have on this. As I'm sure it will all turn out fine.
Perhaps just reassure him that he has done the right thing telling you and the teacher and now the neccessary steps are being taken and he can relax, as it's all being taken care of. Empower him.
Explain that 'buddy with the knife' was very silly because now he's in big trouble! And everybody will be watching him to make sure he is staying away from your child. And then just follow up with the school on this, to make sure they are taking it as seriously as your son is.
Best of luck xxx