The general answer to your question is yes, he is still liable. As long as he is on the lease and signed it, he's liable. Unfortunately you are jointly liable. This means that if he is not willing to front the money, which I'm assuming is the case, you would be liable for covering the whole rent to the landlord and then you would have to sue him for what he owed. This adds a lot of stress on you and it's a long process that you might not get resolution from. My recommendation would be to talk to your landlord and find out what it will take to break the lease. They might tell you you have to pay certain fines or find someone to take over the lease. Both of these options would be easier than trying to hold a co-signer responsible month after month. If you have to pay fines to get out of it, you could always try and recover those in small claims court.
My best advice under property law is to just cut ties for yourself, your landlord may understand if you explain the situation.