Both sets of my 3 year old's grandparents live out of state. When they come to visit they typically stay for several days at our home. Last time his grandmother was in town my son sternly told her to "go home" and "don't say that grandma" which made his grandmother cry. I was very stern with him and made him apologize but a few weeks later his grandfather was in town and my son told his grandfather similar things. What should I do to discipline him?