It's really common, and a change in food often does the trick! But make sure you have the vet get a good conclusive urine sample, because the kind of food he needs depends on exactly what kinds of stones are in his urine.

Get him to the vet right away, and find out what kind of food he needs. Meanwhile, make sure he's got plenty of fresh water to drink, and offer him some canned food because that's got water in it too.

Love, Columbine