Hi So-Sew,
I would definitely ask the doctor about having the family tested to see if someone is a carrier.
I would also be asking the teacher, principal and the school nurse to see if there are others in his class having problems as there could be a carrier in the class.
With regards to the toothbrush, I would simply boil his and everyone's brushes every day until past this episode of illness and then each episode of future illnesses. It does seem to help.
I would also start your soon on larger doses of vitamin C. Get him eating some oranges and other fruit as well. I would also get him a good daily vitamin. I would ask at the local natural food store about an immune support vitamin to help improve his immune system.
Good luck with getting your son back to feeling better soon.
Pam M