You may not like what I have to say, but I'm gonna say it anyway:
It's absolutely crucial that you ditch the dry. It has no place within a 10km radius of a cat with urinary tract issues. Doesn't matter what kind it is or who makes it or how much you pay for it, there is no such thing as a good dry for cats, but even more so in the case of a cat that's overweight and prone to crystals.
Originally Posted by dbg10
The others eat a mixture of canned and dry, the dry only because I can leave it out for hungry cats during the day.
What about leaving wet food out instead? Or getting timed feeders so they can have a wet food snack in the middle of the day? It's also not the end of the world if your cats get a bit hungry for a few hours. The only way you're going to get some weight off your fat guy will be if you stop the kibble free-for-all.
You've probably already read these links but it might be a good idea to give them a refresher: