Messy Max w/attitude

Question
I have a male adult housecat with an attitude. We have had Max almost one year and he has never NOT made his liter box.  The past week he has gone everyplace BUT his literbox.  No change in diet, but the barn cat has found his way to the house and they bicker, hiss and CRY at one another thru the exterrior french doors.  They get into some bad spitting, lunging at each other fits.  Would this be stress?  I shooo off the barn cat whenever I see him, and try to reassure Max of how much we care for him but nothing has worked.  Am I off base here?  Any imput would be much welcomed.  Thanks.

Answer
a trip to the vet is in order first just to rule out urinary tract infection. if he is perfectly health and has no infection, then the reason he is not using his box is that he is marking his territory. you will have to make sure he gets NO contact with this other cat at all. im not sure how you will do that,but thats what it will take to calm him down.