I've had similar problems and had to get a behaviourist in eventually..... Sounds like he is not just having a pee - but also marking things with his scent. From what I understand, this is a security thing with cats - the more they smell their own scent, the more safe they feel in their own home. This links in with the other answers - sounds like something is upsetting him - could be your other cat, could be that he has looked out the window and seen another cat in his garder - his territory. Make him feel as secure as possible and make sure u remove his scent totally or they top it up! A good product is Petfresh but quite expensive - well worth the investment as the bio wash powder/surgical spirit suggestion never works.... Good luck.