What kibble do you feed? Could you post the ingredients? Here's a link to a website that shows which foods are low oxalate (good for a dog who gets calcium oxalate stones), medium oxalate (should be limited), and high oxalate (should be completely avoided):
http://www.branwen.com/rowan/oxalate.htm
Check the ingredients of the food that you feed to make sure that they are low oxalate. If they're not low oxalate, then you shouldn't feed that food.
Also, if you decide to feed the kibble u/d or a kibble that's low oxalate, make sure that you add equal parts water to the dry food. So, if you feed 1/5 cup per feeding, add 1/5 cup of water to it. Drinking lots of water is not enough. Water needs to be added to the food as well if you're not feeding canned.
The u/d is supposed to work for calcium oxalate stones, so if you decide to go with that food, it's better to feed the canned not the dry. The dry contains ethoxyquin and it doesn't have enough moisture for a dog who is having stones, even if he drinks a lot of water.