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This sounds serious enough to warrant attention today, I think. If he is having difficulty urinating, it could turn into a blockage very quickly. It is hard to tell if he's having difficulty defecating or urinating at this point but urinary blockage is by far more serious. I think taking him in today is best and there is really nothing to watch for other than his ability to urinate. If he is able to urinate even a little bit, even if it is outside the box, then you still have time. However, the sooner this is taken care of, the better the outcome usually. Also, remember that cats that have difficult time urinating are typically in pain so this should be a factor too.