I've never fed the recommended amount, my dog only got 3 cups a day from 8 weeks until recently, he's now 11 months and is on 2 cups a day. He's a little bit less active ATM because our herding training is on hold for a few months till it cools down a bit. I also supplement him with raw a few times a week with whatever I can find, this week he's getting whole game hens.
How old is your pup? If he's a puppy and still growing I wouldn't worry about trying to put weight on him, it's better for growing puppies to be on the lean side for their joints. Possibly if you have some pictures showing him from the side and from the top to see what kind of shape he's in would help... It's hard to say yes he needs to put weight on without seeing him.