I would start informally by talking to other EC members. Stay calm and reasonable but firm and clearly stating that you are angry and that this is not acceptable behaviour. Then, depending how that goes, write a letter to the EC as a whole. If directed via the managing agent that puts another person in the loop and may have the correspondence on file. This might be enough to have the EC member pull his head in.
As important as dealing with the EC member’s inappropriate behaviour is, it is also important is that your son sees you go in to bat for him.