It's not necessarily alarming, but it's definitely something to take seriously. The fact that he's still drinking and playing is a good sign, except with cats, even an appetite drop that lasts can turn into a problem really fast. Three days is when you actually need to act, not wait a week.
Before you rush to the vet (even though you should go anyway), try something simple but that a lot of people forget: change how you're presenting it. Not just the brand, I mean the temperature and the setup. Some picky cats refuse kibble at room temperature, so try leaving it barely warm or even a bit cold. Put it in a different bowl, in a different spot. Persian cats especially can be fussy about that kind of stuff.
Now, it's still possible he's just sick of Royal Canin, but you can't really know without a vet's opinion. It could be indigestion, a dental issue, a urinary tract infection, or just him being difficult. The thing is to get it checked quickly before it gets worse, especially since he's refusing to eat regularly.