My dogs do well on Zignature whitefish or Nutrisca salmon, I think the Nutrisca is more bland, you can find it at chewy dot com.
I never have used probiotics, never got into that trend. In fact, I would ask your vet if you should discontinue it?
PS: The prescription foods go through a process so the pup will not react to any ingredients he is sensitive to, even chicken.
The two I mentioned have no chicken, no potato, no grains. They are not prescription, however they are limited ingredients.
I have also heard good things about Pro Plan Focus salmon for sensitive skin and stomach.
No chicken, no potato, it is not grain free, but I believe it may be appropriate for large breed pups? Check it out.
This site has a lot of good info http://skeptvet.com/Blog/
