Hey, don't know too much about nutrition in big dogs but I would definately stick to your breeders choice of food for a good few weeks. Definately make sure he always has water avaliable to him 24 hours a day, especially in this heat
Royal Canin is supposed to be good food but i'd still stick to your breeder's food for a while, and while he's still on that, you can be doing all the research possible ready for whatever you choose for him