I would agree with whoever told you prescription diets are not best and cutting out dry food will be more effective. Dry foods are low in moisture (obviously) which increases the chances of a repeat problem. Hill's prescription diets are a load of cack in my opinion, the first two ingredients of their C/D food are Brewers Rice and Corn Gluten Meal, not the best primary ingredients for a true carnivore. What food(s) are you feeding at the moment?

My advice would be to switch to a raw diet which is a natural way of solving the problem, all bases covered in one fell swoop. Alternatively find a high quality wet food with a quality protein source, no grains/cereals and if possible a reduced magnesium content in comparison to the current diet, feed small meals frequently and have fresh water readily available. You can also mix extra water into the food to up his moisture intake.