I'd suggest a second opinion, with urinalysis done by an in-house lab, which would probably mean bringing him to a major veterinary hospital that doesn't have to send the samples out for testing elsewhere.

Your vet needs to know what kind of crystals they are to know what to do.

In the meantime, there are a lot of different ways to encourage a cat to drink more water and help dissolve the crystals. A lot of cats really enjoy drinking from a water fountain that keeps the water moving. My little scaredycat was actually afraid of a fountain, but turned out to vastly prefer drinking from a glass bowl in the living room to drinking from a steel bowl in the kitchen! You just never know.

He now also gets a joint supplement called Cosequin, that turns out to also be good for urinary complaints - and HUMANS were the "guinea pigs!" Some smart doctor noticed that female humans who took glucosamine and chondroitin (the ingredients of Cosequin) for their joints stopped having such frequent urinary problems. It turns out that glucosamine/chondroitin strengthens the protective glycosaminoglycan (GAG) inner layer of the bladder and urethra, so cats who've had urinary problems in the past often get joint supplements because they help with that too.

Love, Columbine