First you should contact him in writing and make sure that you provide him with your new address. This will set in motion a thirty (30) day window for him to return the security deposit. If he fails to do so, you can file suit (i.e. sue him) for two (2) times the actual deposit. Such a law suit can be started by filing a Complaint with your local District Magistrate. The process is relatively straight forward and you are not required to hire an attorney to do this.