That initial lease will be legally binding. The only way to get him to leave the premises is to get him to sign off on his rights to the apartment to you. I believe it's called a lease reassignment. I had to do the reverse with my XAH: he agreed to take on the lease while I surrendered my portion to him.
However, this may not hold where you live. Have you contacted the local tenant board to find out what you can do? Also, if you're on good terms with the landlord, I'd talk to him asap.
Finally, if your research yields nothing, I'd see about a legal consultation.
If you don't want to go through the trouble, then consider taking your stuff out and leaving yourself. This is also something you'd want to discuss with your landlord, as you'd be breaking the lease before it is up.