The most common causes in a change in the odor of a neutered cat's urine are diet and urinary tract infections (cystitis). However, diabetes, hyperthyroidism and other hormone disorders, impaction of the anal sacs, inflammatory bowel disease, kidney disease, excessive heat and anything else that may cause Bubba to become dehydrated.
Does he visit the litter pan frequently? Is he urinating outside the litter box? Does he seem to drink a lot or more frequently or has he begun shedding more than usual? Does he have increased dander and an oily coat? Are his tail and lower back greasy? Does someone in your house feed him treats or can he get into the garbage? Is the odor accompanied by a darker color urine? Orangish urine generally indicates blood and can be serious.
A good idea would be to see your veterinarian with a urine sample in hand. I have had luck obtaining urine samples by just covering the bottom of the litter pan with a layer of litter one granual thick. The tub bottom clearly visable. You can tip the urine out of one corner into a small (disposable) tub with a minimum of clay detrious.
Best of luck.