I bought a small can of "urinary health" cat food. I took a small smidgen of it, added water to make a thin soup, then mixed it into some of the awful Hills S/O and it totally worked. I doubt that it will work for very long or for actual meals (he is too damn smart), but I am desperate to break his hunger strike!
Looking on the side of the can, I don't really know what makes it "urinary health formula". I don't know what makes any of the "prescription" (they're not really prescription, that's just a buzzword) foods different other than reducing the nutritional value and adding acidifiers. This is why I'm so mad.