Sorry you're going through this I'm pretty sure it's the food, everything you write points at it. It can very well be the beef if he was on a different protein before. I'd probably switch back to what he was on before if the vet said the bald spot isn't anything to worry about... as far as I know "Taste of the wild" is a very good food.
Is the bald spot big and does it grow or change? You could maybe try to put some coconut oil on the food and also rub it on the bald spot if it's still there, it might help