How does he look? The side of the food bag has a suggested feeding table, and that's a really go place to start...but after that you need to base how much you feed your dog on the way he looks. If he looks thin-add a little more, if he looks fat-take some away.
To be honest though, 2.5-3 cups sounds like a lot of food to me. I haven't had a puppy in years...so maybe it's different for them, but one of my adult dogs gets 2 cups of food and the other gets 1.5-1.75 cups of food.
What kind of food are you eating? The better quality the food-the less you'll have to feed.