My girl Nedicks has two types of crystals in her urine and is on a prescription diet for it. Some cats are genetically prone to crystals, while for others it is something that happens as a result of a specific situation, i.e. an infection, dehydration, or food containing high levels of ash and/or magnesium. Either way, crystals can typically be controlled or eliminated by changing the cat's diet, as long as the cat has not developed any large stones (which happens when the crystals stick together). Feeding a diet low in ash and magnesium typically (but not always) controls struvite crystals, but can alter the pH of the urine enough so that the cat then develops calcium oxalate crystals. One of the ways to avoid this is to make sure your kitty is well hydrated so the urine stays diluted. Giving your cat canned food will increase the amoung of water he takes in. You don't have to completely eliminate dry food, but my vet says that by the age of 10, a cat's diet should be at least 50% canned food--sooner than the age of 10 if you have a cat with urinary problems. The prescription food we feed is formulated to make the cats really thirsty, so we don't have a problem with them drinking. However, generally speaking, most cats don't drink enough, because they are desert animals and, well, they just don't drink. So if your cat isn't drinking enough water, you might want to consider getting a pet fountain. We don't have one, but other members here do and they love them--the flowing water encourages the cat to drink, because cats like moving water better than still (which explains why some cats like to drink out of faucets).
It's also important to keep the litter box as clean as possible, to reduce the amount of infection-causing bacteria your cat is exposed to. Even if you use a scoopable litter, every 2 to 4 weeks you should completely change out the litter and scrub down the box with diluted bleach or something else that will kill germs (but don't use Lysol or Pine-Sol, or any pine based product, as they are toxic to cats).
Since your cat is male, crystals are more dangerous for him than for a female. Male cats have narrower urethras and therefore are at a higher risk for blockage if the crystals clump together. A blockage is very dangerous, so it's important to keep an eye on him so that you will notice if he shows any symptoms: going in and out of the litterbox without actually urinating or urinating very little; urinating around the house; straining or crying when he urinates. If you see any of these things you should take him in to your vet right away for treatment. If he does get blocked, the only way to remove the blockage is surgery.
And of course, listen to your vet.
Hope this is helpful to you--keep us posted on your kitty!