I feed my 4 month 42lb pup 3 cups per day. She also gets training treats throughout the day.
Canidae has around 470 calories per cup. So 3 cups should be plenty, if he appears to be boney or to skinny then up his food to 3.5 cups/day.
Although I would NEVER feed my dog purina, they do have an accurate scale of how your dog appearance should be:
dog weight chart - The Purina Body Condition System