Except that a landlord can enter the premises in an emergency situation and although the laws generally mean an emergency with the apartment, dealing with a neglected animal could be enough.  I am assuming that your son is an adult and I am sure that this situation is difficult enough without other obstacles, but keep in mind that the landlord is just protecting your son's belongings here.  So if you can not get your son to sign a Power of Attorney form allowing you to act on his behalf - and then you can enter his apartment - then you may need an emergency court order to do so.  Good luck.