My boy, Augustus had really severe struvite crystals. I fed him Hills S/D dry and it cleared his crystals up in no time. I am now feeding him Summit, which helps prevent crystals. The main cause of crystals in cats is Ash in their food. Too much ash causes acidic urine, which causes Struvites. Too little ash causes alkaline urine which causes calcium oxalate crystals. As long as you keep him on a high quality food, his crystals should subside. Auggys came back a couple of times, thankfully without a blockage. Hope he gets better soon!

((((HUGS))))