What Causes Severe Lower Back Pain In A Person Who Consumes Less Water?

My husband has been having severe lower back pain since early this morning. His New Years resolution was to quit drinking soda. My husband drank very little water before and would drink a 2 liter of coke a day. For a week he's had no soda. Only drinking water. Could he possibly have a kidney stone from all the water flushing his kidneys?