
Something came back to me from the depths of my memory when I read your post. Forgive me if I'm wrong, but this might work. Clearly a dog this size eats from a very big dish, so try placing a couple of tennis balls in the dish with the food, or something bigger if the dish allows or you worry the tennis balls are too small from a safety point of view. He ought to spend a bit of the time shifting the balls, and therefor slowing down the eating.
Dawn R.