Based on your response, it appears that the ex-roomate could very well be liable for the damages that you incurred as a result of his breach of the lease.
Damages:
Eight months rent (I assume Dec was already paid when he moved out) would make $3,300.00 (8 x .5 x $825.00). I assume the difference would be in utilities or other 'shared' billings.
Other issues (assuming you take legal action):
1) He will obviously claim that his breach was your fault and that he was forced out. Would you be able to challenge these defenses?
2) What about the security deposit?? How much, if any, was paid by him?? He could certainly claim his share of deposit. You can't collect both; unpaid rent PLUS keep the deposit.
3) He could claim that you were wastefull in using utilities and that he shouldn't be responsible for YOUR waste.
4) Finally, and most potential problem, is the mitigation of damages. You have an OBLIGATION to try to mitigate the damages. Just saying that you have checked at school and work will probably NOT be sufficient. You will need to show a proactive effort to find a new roommate to share the expenses. This would include ads, notices, placement agreements, etc. What do you have to defend your position that you were agressive in trying to mitigate damages since January???