Some of those veterinary diet foods are pretty bland for dogs, and some just don't find them very tasty. I am sure the diet food is specially blended with everything he needs, and is the best plan. But it is common that they show little interest in their diet food.
If I were you I would call your vet, and mention the prescription diet, and ask if there IS anything you could maybe add to the food to make it a little more tasty and palatable. Maybe even something simple like a little gravy or meat/fish stock could help. But ask your vet first.
When my dog had to have a special diet, I did speak to our vet over the phone, and asked him if there were any other foods at all that she was allowed. The vet did mention a couple of things, which were safe, and that did add some variety in her diet, and make mealtimes a bit more interesting.
(But she was a pretty greedy girl, didn't mind her diet food, and wolfed down anything at all!)
To correctly and adequately treat your dogs urinary calculi, you first have to know what kind of mineral accumulation(s) you are dealing with.
Canine bladder stones come in about six different types:
ammonium phosphate (these are what are known as struvites)
uric acid (ammonium urate)
When you find out which one of these substances your dog's body is producing you can develop a diet that will prevent the formation of that substance.
Whatever the mineral that is responsible for your dog's stones, THE most important thing you can do to prevent the formation of more stones, next to feeding the correct food, is to make sure your dog drinks LOTS AND LOTS AND LOTS of water. You also want to be sure your dog has the ability to go outside and relieve himself several times a day, because holding in the urine while waiting for someone to come home from work to let him out is just as detrimental to the dog as eating something that is going to contribute toward the formation of mineral deposits.
They need to be able to have fresh water available at ALL times, and they need to have access to the outdoors so that they can go when they need to instead of holding it until they are given access to the outdoors. Of course, in a perfect world, all dogs would have on-demand access to the outdoors, but for a dog that is prone to forming crystals, it is IMPERATIVE that they be able to relieve themselves at any given moment.
Meat is safe to feed without worry about forming crystals. Not bones, NOTHING with bone in it should be fed. But meat can be given, so CHECK WITH YOUR VET FIRST, but mixing the prescription food with meat should make it appetizing enough for your guy to eat.
How large a dog is he? If he's a somewhat small dog you may get away with mixing his prescription food with the 100% meat baby food (it's like meat pudding). You should only need a jar or two, depending on how much dry you have to feed him, to cover all the dry food. You will probably have to mix it around with a fork until each piece of dry has some of the "meat pudding" on it to whet his appetite but my guess is that that should work. Again, PLEASE check with your vet to make sure this is in his scope of treatment before just giving it to your dog.