I don't know how old your son is, but assuming he is in elementary school, it depends on what the complaint is and how comfortable your child is with the teacher. It also depends on the complaint. Is it a valid complaint? Does it truly need to be addressed or is this one of those times when you can prepare your son to deal with similar issues on his own. If it's a valid complaint, I would definitely begin with teaching my child to advocate for himself. Prepare him with the words he needs to communicate his concern adequately and respectfully. Then see if there's improvement. If no improvement, then I would speak with the teacher about it.