Interesting. Usually the parties that is doing negative SEO is not the 2nd party (because he know that you would know) but could be the 3rd party, that is neither you or this "big fish". Most likely it is someone you would not know (because the 3rd party knows that you probably would not know).
Just a thought, a 3rd party could also be interested in seeing two of his competitors kill each other with negative SEO. It's also a common tactic in intelligence to "let our enemies kill each other".
Just make sure that it is really him (with proofs and such) before launching a counter-attack.