Need Help Choosing a Cat Food

Greystone is a 9-10 year old neutered male persian. He had a severe UTI two years ago and nearly blocked. Luckily, quick vet care prevented this from happening. (At the emergency clinic of course, I've never known a cat to develop a bad problem during normal vet hours ) But anyway, we cleared him up and the Vet put him on Science Diet CD. I know this isn't the best food, but we wanted the struvite cyrstals in his urine and bladder to clear up.
Well, it's been two years and the crystals are still there, albiet a little bit smaller. He hates the science diet. I want to get him on a better food, but I have gotten conflicting advice about what magensium/ash levels and I don't want to hurt him.
He eats an entirely dry diet. We tried the CD wet and he threw it on the floor if possible and if he was served on the floor, he rejected it completely. I can't afford to pay three dollars a can for it to sit there and go bad.
Any suggestions? He has a good appetite.

I had virtually the same experience with Tink, except that he's only 4.

First, if you haven't already, buy him a water fountain. He'll drink more water and that will help. Tink absolutely hated canned food, but when I tried Natural Balance (which has the same mag/ash levels as the Science Diet prescription), he actually liked it. It gave him severe constipation though--which is a genetic thing for Siamese (he's a blue point) so I had to take him off of that. Grain free or almost grain free seems to be the way to go as far as the mag/ash, so now Tink eats mostly Wellness canned (with some Wellness Urinary Tract Health kibble on the side for nibbles) and sometimes Merrick. Both are close or better to the Science Diet but with much better ingredients. And as a bonus, now that he will finally eat canned, when he has to have some medicine, I can hide it in his food (even the vet can't pill hiim--poor baby had to have shots for tape worm instead of pills...)

I use this for Fang
http://www.belfield.com/products7.html
and feed a good quality wet supplemented with dry.

Our 2 are big fans of Merrick and they have better coats than before I started the wet food in their diet. I give wet twice a day with 1/2 cup dry for them to share. I also changed the dry to Orijen (grain-free). Bijou was a hefty 18.5 lbs and is now down to just over 17 lbs.

FIRST talk with the vet about what % of magnesium , calcium and phosporus your cat needs to not get crystals.. and the Total ASH not to EXCEED ...... IMHO and what I have learned from the vets Grain free IS NOT ideal in a dry form ... some of the grain free wets have fine% ... My vet actually said the only time she does NOT recommend grain free is UTI issues ... yes there are foods OTC that might work in dry and wet , wet preferred as it is a bigger group and easier on kitty as it is closer to natural prey