Switching food in our puppy

Our breeder uses Eukanuba Large Breed Puppy and of course would prefer he stay on it.  As adults she uses their performance food which is  30/20 which imo is too high in fat unless the dog is working/running/agility etc everyday.  She said it works for them for years now.  He just isn't into his food.  Grazes on it for hours rather than eating it all at once.  Puppies we have had always lived for their meals and enjoyed it.  So I got a bag of Wellness Large Breed Puppy for him and he is nuts for it.  Only putting a tiny amount mixed in his Euk had him eating the whole bowl;  He is actually begging at the food cabinet all day long for more.  I plan to transition very slowly to avoid a problem.  Time will tell if he tolerates it.  I assume the calcium level in this Wellness is acceptable for an Irish Setter's growth?  Anyone know more about that?  He spit up a tad bit of foamy bile this morning which is probably from the introduction so I will go even slower now with it.  The breeder felt I should add canned to the Euk instead and leave him on it.  Flavoring it isn't what I wanted.  I want him to like his food.  There are enough good ones out there that I will find one he likes that likes him too!