Your problem is that under the law each of you has the same right to sublet or assign your rights under the lease unless the lease agreement you have with the landlord prohibits that or you had an enforceable contract between the three of you that would have prevented that. My guess is that there isn’t anything in the lease nor any contract among the three of you that prohibits his assignment or sublet of his rights in the lease. In that case, the roommate’s sublet or assignment is probably enforceable and you may find you cannot prevent the new tenant from moving in.