My personal opinion is that it's now time to stop being defensive and justifying things to this guy, and give him a taste of his own medicine. It seems to me, he views you as a "soft target" that will be easily intimidated, and perhaps he can get you to remove your content ebook etc so that he doesn't have to compete with you for sales... I assume you both compete in the same market for the same customers... every one you get is one he probably won't.
But that also works the other way, every one HE gets is one you won't... and you have of course kept all emails from him, right? Especially ones where he states that things are copied word for word. And as you can prove that your content and ebooks came first, then any copyright lawyer is going to have the proof they need that he in fact stole from you and not the other way around.
I would be pointing that out to him, and advising that you are considering putting all emails, screenshots and so on into the hands of a lawyer. A website like this one might be a good place to start tho it costs money, about $50-$60 ( I went thru the steps to find out for you). https://secure.justanswer.com/
While I hate the thought of spending money because this guy is being an %$#@! maybe it would pay to have a definitive answer for when you email him, to be able to say "I have contacted a specialist copyright lawyer over this matter, and not only are your claims completely groundless, I have been told I have a good case to pursue you for copyright infringement, and sue for damages" Or whatever it is that you get told by the lawyer.
See if he changes his tune after that.