For Calcium Oxalate stones, you need to prevent spinach, green beans, beets, any high oxalate foods. You'll need to pick a food that is low in phosphorus, not necessarily low in protein, if it's a high quality protein w/low ash, then it should be fine. You can add omega 3 fatty acid oil for optimum kidney function (although someone said there's no proof of that). You might also want to add moisture to his diet by adding hot water to soak the kibble. It will dilute the urine. Diluted urine will keep the crystals from forming. You can also add some canned food if your dog prefers it that way. Periodic urinalysis is needed. This is what I do for my 2 that had struvite crystals, it has not come back.
RX diets are specially formulated & scientifically tested, but it's not always needed in case of stones. But if it's a recurring problem even after the diet change, then RX diets might be the best route or an option to try.
This link is very informative:
All About Oxalate Bladder And Kidney Stones In Your Dog And How To Manage Them