Well, what works for obese humans? Body composition is a result of both lifestyle and diet. Mostly diet I think. If your dog is massively overweight there's probably an issue with too much food being fed.
Well, one of my male dogs suffers with allergies something fierce. I've been giving him Benadryl to help whenever he gets flair ups for years now with no issues! Maybe you could look into that? Here's some information about Benadryl for you...