Has he been tested for a UTI? That is quite a good possibility, especially if he is on an all dry food diet of low grade food. What do you feed him? There are a load of things you can do to prevent a UTI from reoccurring but you need to find out if that is the problem first. If that is not the problem, add more boxes, try different litters, keep them spotless, buy Feliway, retrain him by confinement to the bathroom, laundry room, bedroom for awhile. A week or so should do.