I believe that your roomate is mistaken but your landlord understands the situation perfectly. The problem is unfortunately yours.
This is a great question. While I have not handled one of these, my understanding of the law in NY (and probably other states) is that a roomate can evict another roomate for eviction and damages if the roomate does not pay rent. He has breached the lease by not providing rent that he is responsible for under the lease. Legally, you are entitled to recover from him the amount that he has not paid. In fact, you may likely recover any attorneys fees and damages from him, especially since there is no basis for a good faith dispute and they should have been agreed to in a standard lease. I'd ask for punitive damages as well.
It may never go this route, but if I were you, I'd read him the riot act for eviction. Sending something like a 3 day notice for the rent (you can find them online but I included a sample below, and they will be available here very shortly). Send it certified return receipt. It that doesn't work, then it's that last notice giving him time to move... or else the marshal will move him.
Most of the time if you look like you mean business, it will prompt the action you are seeking. It's silly for him to think that he could get away with this ridiculous logic!!!
3 DAY NOTICE TO TENANT
TO: Tenant Name and Address
Tenant of the Premises described as _________:
PLEASE TAKE NOTICE that you are justly indebted to the Landlord of the above described Premises in the sum of $_____ for rent of said Premises as follows: $____per month for ____, through ____.
You are required to pay said rent on or before the expiration of 3 days from the day of the service of this Notice, or surrender up the possession of the said Premises to the Landlord, in default of which the Landlord will commence summary proceedings under the Statute to recover possession thereof.
Dated: _____