From what you describe it sounds like the person who wrote the threatening letters was the instigator in this situation. And yet there is usually at least two sides to every story. What does your son say when you ask him "What's your theory about why this kid is giving you a hard time?" Your son may not know or he may not want to say, but there's something going on that's worthy of investigation. You don't say how old your son is, I'm getting the feeling he's at least in middle school if not older. Tell your son that you are concerned and you'd like to work with him to figure out what to do to stop this aggression. I suggest the threatening letters should be brought to the attention of the school.