So if I follow you right, he is having the problems with the chicken and rice formula and you haven't tried the salmon one yet? I didn't think the salmon formula comes in a puppy formula but that might be a decent option if it does. Eukanuba has good puppy food so that is a good option. As far a shedding goes, all goldens do that. Might be just the time of the year.
I would ask you vet for his recommendation.