Roommate breaking lease early

[Curtis Towers]

Hello.

I have a roommate that has notified me he is going to stop paying me and move amount immediately because he is having some qualms with other roommates. The lease specifically states he must give me 30 days notice prior to moving out. I was not able to address his concerns as he did not notify of any issues he was having. I have a $700 refundable security deposit from him and the rent he would owe me for the 30 days would be $900. Can someone tell me my options here?

Thank you