I assume that you've ruled out mites that live under the skin or some oher problem that causes the itch like fertilizer or pesticides in the lawn which get on the feet then the dog licks and now the problem is on the mouth or gets on the stomach from laying in the grass.
If it is allergies it might be from the food the dog eats. Try a single source food. Also, sometimes dogs have an allergy to GRAINS. Most dog food has grain in it ex. wheat or it's by-products. Our dog has sensitive skin and generally scratches. He gets hot spots too, that's another problem- think diluted hydrogen peroxide and apple cider vinegar- spray on hot spots. We have been switching him to NO GRAIN food. You can find them in a well stocked pet store. Canadae (sp?)is a high quality product that has both grain and grain free choices.
We have been buying "Taste of the Wild" which is also grain free. It comes in 3 flavors that I know of: Salmon, Bison, and a Fowl variety. This is a high protein, high quality, excellent dog food. We tried the bison first, then the salmon (has good omegas for the skin) Both our dogs LOVE it. Given a choice of regular dry food and Taste of the Wild in a bowl, thay will pick out theTaste of the Wild food!!! It does cost more than other brands but you are getting MEAT. We have large dogs (100 + lbs)so I understand the cost factor however it costs less than a vet bill. We've noticed him scratching MUCH less as we've made the switch. He's almost completely on "Taste of the Wild" now.
Be sure to introduce the new food slowly, and one flovor at a time, as a quick change of any food can cause loose poop. Try a scoop of the old with a scoop of the new, then less and less of the old. Because the food has no fillers you might notice he eats less and poops less too. Whole meat is more filling and more throughly digested.
Hope this helps. Good Luck.