I'm so sorry to hear this. He may be constipated. Hopefully he won't need a kitty enema. Do you have any canned pumpkin? If so I'd try to give him some if he'll eat it willingly. My RB Pepper had problems like this and he had to be on lactulose every day which did the trick but he hated the taste of it. I know that older cats need more fiber in their diets. Storm once had this problem but now that he's eating plenty of canned wellness every day he's been fine. I hope that everything will go well at the vets tomorrow. Lots of prayers and positive thoughts are being sent his way.