A food allergy wouldn't, by itself, cause the loss of weight or the excess drinking, it would only cause weight loss if he stopped eating because of it. I, a human with food allergies, can attest to that. I have had food allergies that caused vomiting, diarrhea and anaphylactic type reactions - mine run the gamut, depending on which substance it is I ingest. Yes, when I stop eating entirely for a couple days, I am sure I lose a bit of weight, but nothing long-term like he seems to be experiencing.

I am glad you have a vet appointment set up, if they are not able to give you answers, of course, ask for a recommendation for someone else.