In my opinion, the diet has too little meat in it. Not an ideal situation for a dog, especially an allergy-prone dog. Remember, dogs have a harder time processing plant-based proteins versus animal-based proteins. They also can't process carbs as efficiently. Plants is better as a small percentage of the diet - under 40% if possible. 0% is just fine.
To determine how much food to offer... If you're feeding lots of plant matter, you can go by total calories to match what your dog has been eating with the kibble then adjust up or down depending on the effect on the dog (if he's getting skinny, increase the food and vice versa). If you are feeding minimal plant matter, you can go by the weight of the meal. Start with 2%-3% of his body weight daily and adjust up or down accordingly.
My allergy-prone dogs are on a Prey-Model Raw diet. They get plant-based food only for occasional treats.