That's no good. I hope it's not crystals since waiting on a boy with those could be scary.
Are you to try to collect a sample at home? If it's a simple matter of having him in when his bladder if full, if you could bring him in after a long nap (if he sleeps most of the night with you) but before he can get to the litter box, that should work. You can also gently feel to see if a cat's bladder is full.
If your vet has never shown you how to do that, ask the next time you're in since you have a cat with repeated UTI's.
I hope you can get a sample out of him soon.
