I'm Dr. Kris and I'd love to help but first I have some questions.
How long has this been going on?
Have there been any recent changes in food, treats, carpet cleaners etc?
How much benadryl did you give and for how long?
Is the itching worse at any particualr time of the day, after he comes in from outside, after eating?
Is he on flea prevention?
It seems to me since he itches his mouth after he eats that he could have food allergies or he is allergic to the food bowl if it is plastic.
Have you ever tried feeding him a prescription hypoallergenic diet or one with lamb as the protein source? If it is a flea allergy, it only takes 1 flea to set him off and you may not see any but they may there.
You said this all started after you gave him a flea bath? Were any of the dogs that died having any of the same problems?