They often go into "hunting mode" after they eat - it doesn't mean that they need more. It's just a survival instinct because he doesn't know that his next meal is guaranteed.
Having said that, if he can eat 3 fuzzies at once and keep them down, he may be ready for the next size of food up. A good guideline is that the food item should be around 1.5 times the width of the body (not the head or neck) and leave a visible lump in the belly for 1-2 days after eating. When the bump starts going down faster, it's probably a good time to try the next size of food. This holds true until they reach large mice, at which point most Corns won't need anything larger for the rest of their lives.
Remember that as food size goes up, feeding frequency needs to go down to allow their digestion to work properly.
It sounds like you could safely try feeding him one small mouse a week and see how that goes. Don't be alarmed if it takes him *ages* to get the mouse down the hatch the first few times. Sometimes it takes them a while to get the hang of bigger food.