Cosigning for an apartment?

We live in Minnesota. We co-signed for our son's apartment when he left for college. The agreement we signed states that we do not have to sign again in order for him to renew his lease -- which he has done through August 2011. He has also failed to meet his rental obligation three times in 2010. In order to protect our credit rating, we paid his rent for him. He is not going to renew after this lease expires, but I would like to remove myself as the co-signer in the meantime. Is this possible to do? How do I proceed with a legally enforceable method? Can anyone offer Minnesota statute that will support my position? Thanks in advance for any help offered.