Well this gives you the opportunity to find a new housemate without losing money. The situation may be an inconvenience at the current time, but you can end this situation in better standing than had you taken your route.
First, find a housemate as soon as possible. Get him in prior to the following month, that your current housemate leaves.
Second, take your housemate to small claims court for the monthly rent. Here you win a moral victory by teaching him he's not above the law and you are compensated and made whole through the legal system.
Congratulations, things are looking up.
Additionally, since your housemate is a deadbeat, you're probably worried about collecting on the rent and having to have courts garnish his wages. Here's the beauty of the situation, assuming that in order to rent the house/apartment you had to put down either 1, 1 1/2, or 2 month's rent - you already have access to a large chunk of that money. Speak to the landlord and be sure that he doesn't release your housemate's security deposit (in order to protect himself, he shouldn't release the deposit until the expiration of the lease, but due to your state's laws and the legal ability to break a contract given written notice, he may be obligated to release the deposit after X number of days) and if you can, have him access the current damages to the place you're renting. This way you know exactly what your housemate owes in terms of his deposit, as well as rent.
Now go get another housemate and have him put in his portion of the security deposit.
If you handle the situation this way, you really shouldn't lose any money in terms of damages or rent. The new housemate will make up for the other half of the security deposit, you can keep your former housemates security deposit and then sue him for the remaining balance.
bingo, you're all done.
If you have any questions about the way I laid things out, just let me know and I'll attempt to clarify, myself, further.