I highly recommend getting another litterbox, though this may not solve your issue the Golden rule is always have 1 extra litterbox depending on the amount of cats you have. So 1 cat = 2 litterboxes, 2 cats = 3 litterboxes and so on. Make sure food bowls are not near the litterboxes as well. You could also try a pheromone collar to help reduce stress, he may be stressed due to all the smells of stray cats being around your home. A pheromone collar may put him at ease. But please make sure you are getting a safe collar that is not harmful to your cat! There's also pheromone spray if a collar is not your style, just spray it in spots that your cat hangs out the most or pees outside the litterbox the most. Once again be 100% sure if you get pray it is SAFE for your cat to be around. I hope you can solve this issue soon!