Stuff them @Mary Poppins, they have not done a clinical examination of your horse and even if they think they have they are not qualified to diagnose anything! irritation of the airway tissue due to an allergy is quite different to a bit of mucus in the airways and requires different treatment, I am sure your vet listened to his lungs and deemed there to be little/no mucus to clear or she would have told you about it.
And re his weight, I think you are dead right to be following your vets advice, too many people think what is actually a fat horse is normal or 'has good topline' and they don't understand why we want them thinner, I've had it a lot with Jess and Quarter Horse people, because the breed is heavy set and heavily muscled people seem to think being 'chunky' is the same thing and I often get comments about how I keep Jess too thin, but IMO she is still a little on the porky side, so remains on her permanent diet.
And re his weight, I think you are dead right to be following your vets advice, too many people think what is actually a fat horse is normal or 'has good topline' and they don't understand why we want them thinner, I've had it a lot with Jess and Quarter Horse people, because the breed is heavy set and heavily muscled people seem to think being 'chunky' is the same thing and I often get comments about how I keep Jess too thin, but IMO she is still a little on the porky side, so remains on her permanent diet.