I have another hint, for the meantime; you could try using several litter boxes, in different locations. I am not sure if your house is big, but I have heard that sometimes they will go if they can't get to the box. I guess if the cat has a UTI, it could even compound that problem.
Does it look like he is in pain when he urinates? My mom's cat as put to sleep last year, but in her later months, she was bleeding while urinating, and in fact was going in the bath tub for some reason. She had a kidney problem.