Your cousin may have been born with a dysplastic kidney or he may have had an infection called pyelonephritis, which can cause shrinkage of kidney. This kidney needs to be removed only if there is uncontrolled blood pressure resulting from this kidney or there is uncontrolled life threatening infection related to this kidney. One must also evaluate the function of this kidney before deciding to take it out. There are no medicines to recover lost kidney size and or function. The blood pressure should be monitored regularly and targeted at less than 130/80 mmHg to preserve the function of the other kidney.