@connie. I wouldn’t put him on a dry diet at all. For crystal you want to keep the dog as hydrated and his bladder flushed out as possible. That’s the key as well as making the diet less acidic. Maybe get with your vet and ask about a wet option or an over the counter recommendation. If he refuses or can’t give u further info find a vet nutritionalist for other high moisture diet options.
Ideally the Royal Canin will desolve the crystals within a couple of weeks and then a hydrating diet should follow after. Make sure your dogs bladder is staying flushed and he is urinating as often as possible. Every 2-4 hours.
