my 17 year old son told my mother,his grandmother to ,GET OUT OF MY HOUSE

My mother and i have been verbally fighting for years about nothing, just constant bickering back and forth. last night during another heated argument this is what happened - my son was downstairs on the computer and my 10 yr old daughter went down to tell him that we were fighting. he came upatairs and told my mother with his fists clenched to GET OUT OF MY HOUSE then my mother said to him' try and make me'.. who is at fault here? should my son disrespect his grandmother that way or is it okay for him to defend me, his mother?