I have a 4 month old er/MiniCollie mix that always seems hungry!?

He eats Science Diet, as per bag for a 4 month old 5 lb. puppy, 3/4 cup a day. He is SO active, well behaved, crate trained, all the good stuff. But he seems so hungry all the time. Hoovers the floor for scraps and GOBBLES up his breakfast and dinner. Also, time for a new bag, how to I switch foods as he grows?