We are not in KY yet which is probably why no one has replied to you. I am not a KY lawyer, but I am a lawyer (and the CEO of Avvo) and I want to offer some general thoughts regarding your question.
You are correct that it is illegal to open someone else's mail. However, if that person authorizes you to open it, then you are fine. If you are worried about anyone claiming that you are not authorized to open your son's email, you can solve this by getting his authorization in writing.