The stray from outside could be enough to stress him out and cause the spraying. One thing about fostering, is my Pouncer gets stressed with the fosters. I'd love to foster new kittens once one leaves, but I usually have to take a few weeks break in between fosters because Pouncer gets hissy and growly. The rescue told me to listen to his signs of stress because if they had done the same with some of their own cats, they wouldn't have the spraying they have now. Cats are amazingly senstive and the littlest things can stress them out. Good luck with the Feliway and adding a 4th litterbox!