The latest game he is playing is trying to claim we owe back rent. He refuses to accept signature conformation mail, so as to avoid having a trail of the rent we send him. He didn't cash two of our rent checks over the past seven years one in 2011 and one in 2014, then he put two of this years rent checks towards the 2014 and 2011 rents from the checks that he didn't cash so he could claim we owe back rent this year.
If you can prove you sent those checks (certified mail receipt, etc.), that's a defense against a nonpayment case. Even if you can't prove it, you still technically owe the money, but there's two important things here: (1) the statute of limitations for most civil cases is generally six years in NY (not sure if additional notice resets this in a nonpayment case, but I doubt it - that's a good question for an attorney) and (2) if he's using unpaid rent from 2011 and 2014 to argue his nonpayment case, another potential defense is laches, but beware that laches is a complicated defense and you should probably seek an attorney to do this for you.
Disclaimer: this is just a general opinion and not legal advice. I am not a lawyer. For legal advice, seek a qualified attorney.