While there the vet got a sample of his urine (which was mildly bloody. He tested it.
Apparently male cats will sometimes just get urine crystals and that, according to the vet, nate has to be on perscription food for the rest of his life.
The good news is he had no bladder stone. It's all medication and diet change, no surgry, but I'm kind of sad that Nate has a chronic condition. It's going to be tough feeding them separate foods (and yes, the perscription food has a chance of being bad for Iz) since they just nibble at their food.
They gave him a subcontaneous fluid inejction to help him flush the crystals.
I guess I should just feel lucky it's not the bad cat food renial failure problem.