There is no statute that applies to this. You should send him a letter via registered mail, return receipt requested, telling him that you are raising his rent to $X. Simply set it forth as a statement of fact, and don't invite him to discuss it with you.
I'd make it effective as of the first of a month, but give him about 5 days' notice before the effective date (not required by law, but I think it's a good idea).
If you have problems or further questions, contact a lawyer from the Avvo listings for real estate lawyers, or the New York County Lawyers' Association on Vesey Street, downtown. They have an attorney referral service. Some of the lawyers offer free consultations.
I am an attorney admitted solely in NY. None of the answers I submit on this forum constitutes legal advice, even to questioners in NY, and no attorney-client relationship is hereby created.
Dear and they have been renting month to month:
Perhaps I might help. Solve this problem by offering the tenant a new lease for a term of years. The lease you offer may provide a fixed monthly rent, that is remaining the same for a fixed number of years, or you may propose a lease with a schedule for annual predetermined rent increases. You could find a good commercial lease at a legal form printer or publisher. Lease forms are available online. You may also hire an attorney to tailor make a lease which works for you and the tenant.
When you have a month to month tenancy, the tenant only continues in possession if willing to pay the rent. If you do not want to strengthen your ability to receive the rent by a new proposed written lease, you may find that your tenant will challenge you to terminate the month to month tenancy. You increase the rent by sending an invoice for the next month (August 2012) at the new rent. If the tenant pays, then you have a new monthly rent. You are in a stronger position with a new lease.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.