What is a good dry cat food for a kitty who is overweight but is prone to stuvite crystals?

I have a kitty. A big kitty who has been molly coddled food wise due to the fact that he is prone to FLUTD and obstruction. He is almost 8 years old and although he is a long cat he weighs about 20 pounds. Thank goodness he hasnt obstructed in over 2 years but he has obstructed at least 7 or 8 times in the past. I want to get him on a diet of sorts but am afraid to give him food like Science diet R/D because of the magnesium and calcium content and thats what the vet always gives him when I mention his weight. Unfortunately he always obstructs with it. :/So, is there a good food for portly kitties that do not have the additives that can cause obstructions like magnesium or calcium but are healthful and rounded nutrition wise? Are there any organic or natural options out there other than supermarket brands? Thank you! :)