I think, if I were you, I would try another vet if that's a possibility.
Royal Canin is not a good food. There is a very good food available at PetCo called Natural Balance...try the duck and potato, it's affordable and it's made for allergies. I was feeding that to my boyz when Jo started having some skin issues...but...I was also feeding a raw beef diet at the same time so I think he was allergic to the beef. I've taken him off of the raw food and he's back on the Natural Balance with no problems...so far
However, there are many other good foods available too but the Natural Balance can be found at PetCo instead of a specialty store.
When I took Jo to the vet the last time he had several liquid filled blisters on his belly that he would lick and break open and I could feel one or two on his back and shoulder area, his coat seems like it's always oily between baths and he licks his feet a lot although I don't ever remember him having dandruff. Our vet thinks he has food allergies too so she put him on antibiotics and gave me some medicated shampoo called 'Pyoben' to bathe him in 2 or 3 times a week at first and then once a week after he cleared up. I also have a shampoo I bought at PetSmart called efa HyLyt shampoo...it's a soap free, hypoallergenic shampoo for routine cleansing for normal, dry and sensitive skin. The antibiotic he was on is called Simplecef.
Also, if he scratches his ears alot or if his ears smell it could be a yeast infection that has spread. Has he had his ears checked?
Jo is doing better now...I hope you find relief for your boy soon...allergies can be frustrating for everyone involved.