Lease Non-Agreement

Just wondering what my standing is on the following situation.

I live in a 2BR apparment, my roomate at the time got a great deal on a condo and moved out 1 month before our lease expired, and he payed up his half, everything was fine.

I had a friend of mine added to the lease and the first roomate was taken off, my friend was slated to move in 1 month before the lease expired, but due to his lease at his current place, couldn't make it. During that month, he had got a job in a differant city and moved away, leaving me paying the full rent for a 2BR appartment. I still have a copy of the lease with his and my signatures, as well as the landlords. 3 months go by, I have been looking for a new roomate, but haven't found one yet, and my "friend" has yet to pay me any money. I've called him a few times over the 3 months, and he said he would pay what he could afford. I haven't seen a dime yet.

My questions:
Is he obligated to pay half the rent?
Is it my responsibility to find a new roomate?

You have to keep looking but if need be can sue in small claims for rent in the meantime...