You don't have any right to search his or her apartment unless you had a power of attorney from him or were on the lease. Otherwise, even if you fear he is missing, you cannot legally enter his unit and the landlord or property manager would be violating the law to let you in. The police can go in, if they have legitimate reason to believe someone may be dead or injured in there; you have to let the police handle this (but should stay in contact with them), since you, as a private citizen, have no legal authority.