Is a dog nutritionist the right person to see for food allergies?

My dog has food allergies, but all the specialist has done has put him on a fish and potato diet, which he has to stay on for the rest of his life. I would like to try and work out exactly what is wrong and what foods to avoid, but I am worried about doing an exclusion diet on my own, so is a dog nutritionist the way to go, or something more "alternative" like a herbalist?