The recommended amount does look pretty low to me...
Have you had him on a regular dewormer? When I first got my pup, he was really hungry and I had to feed him so much food to maintain his weight. When I took him to the vet a week later, his fecal showed no worms, but when I gave him a dose of the heartworm meds a ton of large roundworms came out in his stool... Ewww! He was happy with a lot less food after a couple days.
f you've ruled out worms, then I would say to go by how he looks. You don't want his ribs showing, but don't want him too chubby either as its not good for their developing joints to have excess weight.
I pretty much decide how much to feed each day by looking at my puppy.. I up the amount if he looks like he's getting slimmer, and if he's looking pudgier, i'll maybe decrease it a little.