Again... NEVER EVER EVER change food dramatically, EVER. EVER.
Evo... actually... ANY Natura product is infinately superior to Royal Canin, but you should never ever change their diet like that, if you dramatically change your pet's diet you are simply asking for trouble AND vet bills
I would suggest you try again, but wean him onto it over a period of two weeks or so, your pet will thank you for it.
The problem with most pet owners is they love their pets and alk to them, but they dont always listen to what they're trying to say...

Think of it like this, when I travel I always make sure to spend the first week eating a mix of native and Western food, every time I have thrown myself into the culture and eaten, say, Indian food three meals a day I have come down with nausea and diarrea, and once a bad case of gastro, which required hospitalisation, and it wasn't food posioning, it was just the change in diet - how bodie ALL our bodies are sensitive, how internal workings are very complex, be kind to them.