Others on this web site can offer you more info on brands of food than I can.
However have you tried Medi-cal Hypoallergenic or medical vegetarian? You can only get these from a clinic and they may? be alittle more in way of $. But it sounds as if your guy is very allergic to almost everything and constantly changing foods probably isn't helping.
I will honestly say that there may? be better foods, but I've always loved medical and always used it with great results for my own dogs.
Luba always recommends yougart, yougart should help.
What about duck and potato? Ask your vet, get a pamphlet on a vet recommended food and compare ingredients and the guarenteed anaylisis (Sp?) with other foods.
As I said others here are more educated on foods than I am, but my own personal experiances have lead me to trust and love medical