When our cats got urine crystals, our vet told us (although I'm sure science diet would be angry) that if you take the ingredients on the side of the bag, go to a regular pet store and find other foods with the same ingredients (making sure, in particular, to find low ash content) they would be fine. Right now my mom feeds them Purina's brand for urine problems and they are all doing great. I don't know how many vets actually come out and tell people this because Science Diet is sold in vet's offices (up here anyway). I would try something else if it were me - maybe he just doesn't like the taste.

Good luck