I am so sorry to hear about your Mickey. I know firsthand how difficult and expensive it is to have a dog with chronic health problems.
What type of stones does he get? Calcium Oxalate or struvite stones? I know struvite stones can often be managed and/or dissolved with special diets.
I found this article interesting. It said males are 75% more likely to get calcium oxalte stones and Yorkies were one of the breeds they are common in.
http://www.2ndchance.info/calculi.htm