Aww, the poor little guy. My cat once had very similar symptoms and I was so worried it was a urinary blockage that I to him to the e-vet. As in your kitty's case, it turned out to be his bowels, not his urinary tract (I had also recently done a food switch). Even so, I was so scared by the experience that I started reading up on diet, UTI, blockage, IBD, etc. It was actually around that time that I discovered TCS!
My cat is just finishing a round of clavamox for a UTI. I notice poo on his bum at first. Turns out he was straining so hard to pee that he would poo instead. He is doing great now. looks like we won't have to do a second round of meds. Make sure your guy pees. I caught my cat so early that the vet said he didn't have a fever yet. I think that you may be dealing with IB from the sound of it. The vet did tell me that with a blockage I would have been able to feel it him and that he would have been so uncomfortable when I put pressure on his bladder, that he would have howled. Maybe try feeling for a blockage. Hope it all works out for you. I always hate it when one of my fur family gets sick over a weekend and the vet is closed.
Thanks for the advice - I felt all around his belly and abdomen area yesterday to see if I could feel anything or if was in discomfort. I had no idea where to feel or if he'd react, but I figured if I felt something hard, or he yowled then we'd go straight to a vet. All felt fine and he just kept wrestling with my other hand. Good to know that was a good thing to do!
There was definite good sized wee clumps as well as some loose (and a couple of solid) stools in the litter tray he usually uses, so fingers crossed that he's fine.