Is he free to curse at you? Yes, the 1st amendment protects free speech. He is not allowed to threaten, harm or intimidate you, which based on his letter, he has not done. It sounds like you have a bad neighbor, but no laws were broken.
Well, the language is a bit updated. But this sounds like the classic German-Texan property line discussions I grew up to know and love. Prince Frederick might be proud!
Anyway, what's your question?