If my son and his wife are having marital difficulties, can my son bring his son to visit us out of state without getting into trouble?

In short I start with saying if no court order is in effect that he has equal parental rights as does the mother. However I would advise that he still notify her of the details of the trip including approximate dates, times, and locations the child will be. Otherwise there is a possibility she could believe the child has been abducted and a parental kidnapping argument could arise.