is it ethical for my brother to go behind my back and see my son's trust attorney?

If your son is being represented by an attorney appointed by a court, and if your brother is the executor of your father's estate, and if your father's Will is leaving something directly to your son (which appears from your question to be the case), then your brother was required to deal with your son's attorney, not with you. You are not your son's representative. Your brother only needs to discuss your interests with you, not your son's interests. As Executor, he is required to deal fairly and honestly with the beneficiaries; if a beneficiary is represented by an attorney, that generally includes communicating with the beneficiary's attorney. That being said, as an heir of your father and (maybe?) a beneficiary under his Will, you are also entitled to your own attorney. You may find it helpful to consult your own attorney in order to get a better idea of your rights and interests with regard to your father's estate and your brother's role as Executor and the related duties he has. Best wishes to you.