What happens when a joint tenant leaves?

I have reason to believe that my joint apartment tenant may leave without paying this month's rent or the fee to break the lease (two months rent). My portion of the rent is current, and I do not wish to move. What legal recourse do I have to prevent myself from being evicted, ensuring he pays his part of the rent/lease termination fee, and protecting my credit? I have heard that even if I have paid my segment of the rent, the landlord can hold me fully responsible for all money owed. Is that true? Is there anyway to prevent him from doing this?
Thank you.

You signed the whole lease, not for half the rent. The landlord can sue you, and you can sue the ex for half. But you're on the hook for 100% -- as is he.