Hello everyone, really need your advise and help ?? I just got my very first puppy, his name is Jasper

!! He is now 3 months old ( had it for few weeks now ) and would really appreciate some advise on the food. I had been feeding him Royal Canin ( terrier puppy food ) as per my vets advice and now my trainer told me to start it on Acana wild prairie regional formula grain free. I got a bag today and came to realize the knibble size is quiet big compared to Royal canin

I mixed it in with Royal canin today to see if he eats it but he seems to pick out the royal canin one and left the acana behind. Should I continue with the Royal canin as he likes it ? for try transition him to acana if so any advise as to how ? The reason I changed the food , its because he has been itching a bit ( he has been treated for fleas and tics and vet checked with no problem ) my trainer says they do that at growing stage but if it happens to be the food than changing the food should take care of it. PLEASE HELP !!!