It depends upon what type of crystals they are.
There are two types of crystals: struvite and calcium oxalate.
You need to ask what type they are. I assume they're struvite, because a diet that helps lower the pH can prevent their occurance. I would call the vet to find out if this, in fact, is the case.
Here is an excellent article describing the issues with crystals: http://www.peteducation.com/article....articleid=2729
Diets for struvite crystals, I don't know. Any "urinary tract health" food would work, and almost everyone makes one. Royal Canine would be a pretty good high quality food.
We have male cats - one has a problem with Struvite crystals, 2 have a problem with Calcium Oxalate crystals.
To get them to drink more water, we bought a Petmate fountain. Those made no difference to our cats (the water pours down a ramp). Then we bought a Drinkwell fountain. It pours out a spout, like a waterfall. They all started drinking more (you can tell by the amount of pee you have to scoop!
). We bought another one so we've got one on each floor.
We also mix a little warm water into any wet food we feed them.
The issue with calcium oxalate crystals is the way they metabolise magnesium and calcium (magnesium affects the metabolism of calcium). We had to put our cats on a prescription diet, and NO MORE SEAFOOD OF ANY KIND as a treat. Our kitties loved shrimp, so this was a bummer. Seafood tends to be high in magnesium. Also, NO MORE GRASS OR FRESH GREENS. These are high in calcium - or some ingredient that affects the calcium. This was a bummer, because they loved cat grass.
I do know that a diet that is high protein and low on grains is best for urinary tract health.
Also, I hope the vet explained to you that you MUST keep an eye on your kitty. Male cats have very thin urethras, and they can become blocked. Switching your cat's diet will obviously help, and may prevent the problem from recurring. However, if you ever see your cat straining to pee and nothing except a couple of drops is coming out, and this happens more than once in a day, get your cat to a vet ASAP. When a male cat becomes blocked, the urine will back up into his system, and in a little over 24 hours can cause him to die from toxicity.
A blocked urethra will not kill your cat. Not getting him to a vet in time to fix the problem could.
Don't mean to scare you - just don't want you to learn the hard way.