If your roommate who has a 12 month lease for the unit you are occupying with you is obligated for one half of the rent for the next nine months. He can possibly seek to sublet it out to a third person to cover his obligation under the lease.You are entirely within your rights to request that he pay his share of the rent for the next nine months and have it paid in advance. I suggest that any agreement reached between him and you as to the balance of the nine months on the master lease be reduced to a written document signed by each of you where a copy of the agreement is kept for future need.