Bigger, more nutritious breakfast and replace sugary snacks with more filling, nutritious ones.
My son (2.10 yrs) goes through days where he is like this, too (although because he has traditionally been a very picky eater, I love seeing him plough his way through big meals!)
Yesterday for instance he had:
Breakfast: Huge bowl of shreddies & cheerios with milk, 1 banana, 1 slice wholemeal toast w/jam, 1 innocent smoothie
Morning: Cheddar cheese slices and wholewheat crackers, 1 apple, 1 satsuma, another banana, raisins, 1 glass of milk
Lunch: Veggie sausages, mashed potato & peas; 1 strawberry yoghurt, 1 fruit puree pot, glass of water
Afternoon: 1 slice whiolemeal bread & cream cheese, 1 pear, 1 'Goodie' cereal bar, 1 glass milk
Dinner: Meatballs, pasta, sweetcorn, greek yoghurt w/ blueberries, glass of water
Bedtime: wholemeal muffin, cup of milk
I couldn't eat as much in a day!