You have two issues here - one is that he MUST be neutered. The other is he is showing signs of a urinary tract infection, perhaps leading into a blockage. A blockage is an emergency - if he cannot pee, his bladder will fill until it bursts, and he will die a painful death. Watch him carefully - if he jumps into the box frequently and only a few drops or nothing comes out, he needs a trip to the emergency vet - immediately. If he's still going then call your regular vet on Monday and insist he be seen - they do take emergency appointments. While you are there, make an appointment to get him neutered. He could have/should have been neutered prior to reaching 6 months old, there was no excuse for waiting. He is not marking territory in his box - he is trying to tell you that something is wrong.