Poor munchkin. My cat had FUS/Cystitis a few weeks ago. It turned out that the stress factor that caused it was a mixture of my fiancee leaving to boot camp and the new adopted kitten. Poor girl was so stressed, she gave herself this horrible condition. She had struvite and bilirubin crystals in her urine.
I was so against giving her a prescription diet because I just cannot compromise on her diet because of this one condition. So I spoke to a like-minded vet and got great info from this forum and decided to switch her completely over to Nutro canned. She has been doing GREAT!!! And she loves the Nutro. Another thing I think helped her and me was that I read to her from "A New Natural Cat" about FUS and what the possible stressors could be. Yes, I read to my cat. I told her that I was worried about her and that I knew she was worried because I was worried and because Roly was gone and everything else, but I would try to feel better so she could also. It must have worked!!!
What were you feeding him before (sorry if you posted that, I'll go back and look). Canned helps with crystals...and adding Vitamin C does as well. I forgot to mention I have been adding Vitamin C to her food. It helps with stress. What makes Nutro special for this condition is the DL-Methionine in the recipe. This helps to acidify the urine and dissolve the crystals.
I found a great site with awesome info about this condition and can't find it. Send me a PM if you're interested and I'll see if I can email it to you. I'll most likely have to scan and email it.
Good luck with your baby!!!!