In my opinion, I don't feel NB Duck & Potato has enough meat. It also seems very dry compared to other kibbles and Snick would not touch it when I tried it ages back. Other kibbles seemed to smeel and look more meaty with more dense kibbles and more oily.
California Natural by Natura is also a great low ingredient kibble but it has grains. Many higher protein kibble is grain free. Fromm has a Surf and Turf which is Fish based, can they have fish? Acana has a grasslands formula which is lamb based. I'm surprised the vet specified exactly duck and potato.
They do seem young to have allergies determined.
Good luck
-Snick & Viv