It should be "I bet he is AS hungry AS a pumpkin by now." The correct grammar pattern is "X is AS Y AS Z".
Anyhow, we don't use any of these idioms in my part of the United States: Western Massachusetts. Like Dr. Hall, I also have never heard any of these. We would say, "I'm so hungry, I could eat a horse." here.
The idiom "as hungry as a bear" makes sense to me though. "As hungry as a pumpkin" just sounds silly since it isn't a predator or an animal like a pig or a hippo that eats a lot. Actually, you might even be able to say "I am as hungry as a hippo." We don't use this idiom here but people will easily understand it and they will be reminded of the childrens' game "Hungry Hungry Hippo" :)