What kind of bedding do you use? Just curious. The next time you have him out to clean the cage, empty everything out, and pur some white vinegar in and let it sit for a minute, then rinse it out with water, then you can use soap and more water. The vinegar is mildly acidic and so can help remove some built-up residue.

Then plan on cleaning it twice a week, and see how it goes!

P.S. I moved this into Pet General - Dog General is for dog stuff!