I don't necessarily think it is unwise to tell your son that you are on his side. Just point out theres not always anything you can do about it.
If you have spoken to the teacher several times, and nothing has changed, you have every right and reason to go to the principal.
At the same time, teachers talk, and you don't want to be seen as "that" parent. You really have to weigh whether it is going to help or hurt in the long run