My son who is 24 and lives in my home has a daughter 1yrs old the baby mother will not let me son see the daughter unless he agrees to her terms.

If the parties were never married, one of them needs to file a petition to establish paternity in court. They can do that by either signing a vountary acknowledgment of paternity OR file a formal paternity case at the court house. The court can order D.N.A. testing, to show whether your son is the biological father. Once he is established as being the child's father, the court will also hold a  hearing to determine his custody and placement rights. He will also be court ordered to pay support. If he already has been established as being the child's father and has custody and placement orders in place that the mother is interfering with, his remedy is filing either a motion for contempt or motion to enforce physical placement, so he can see his chlid without ongoing interfereence from the mother.