That was so far out of line, I don't know where to begin! Did you ever notify school administration? I am wondering how they did/would react.
Did your son get skipped into k? When did that happen- was it very close to "graduation"? I am trying like mad to come up with some scenario that doesn't make this guy out to be a nutter, but I really am not having much luck.
I like that way he says he was going to threaten and coerce, but he changed his mind. As if that letter isn't threatening! I might even have brought it to the police. A restraining order would be a nice touch.
hugs to Cathy, and I hope that meddlesome Dad is done causing trouble. Please don't let people like this dictate what your son does.