I don't think you did the wrong thing but I would start teaching Ethan how to stand up for himself or this could end up being an ongoing problem. Try to teach him how to use his wit to outsmart the bullies.
A course in self defense wouldn't go astray either it will give him the self confidence he needs to back himself up without the need to resort to violence.
It's unfortunate but I don't think there is much schools can do to prevent bullying. For all the stuff theytry if a bully wants to be a bully they will regardless.
JMO