Generally speaking, no.  A landlord is entitled to the keys to his building.  First off, your husband needs to negotiate a lease. He has a month to month but he needs a "regular" lease (I believe commercial would do here).  How he wants to negotiate the rent is up to him.  I would seek legal help with all of this part.  Once you have a lease you have protections as well.  Like against unreasonable entry by the landlord.  You have always had a right to call the police on the son if you caught him there.  He is a trespasser.  I am sure that the landlord will try and say that he gave permission but he really can't. Ask the lawyer about that as well.  Good luck.