Intake of soda pop of all kinds has been associated with obesity and weight gain. This includes intake of diet sodas and colas. Colas in particular have a large amount of phosphorus (that makes the color dark) and this is also associated with heart disease as well as obesity. I would suggest that he try just plain water in the same amounts and see if he feels better and loses some weight.
Obesity and soda intake are both associated with increased incidence of chronic kidney disease (CKD) and progression of CKD.