
Another thing to consider is the amount of treats that are being used for training rewards. You could use some of his kibble, taken from his food allowance, to use as treats, or if you are using higher value treats such as cheese or kibble you could reduce his food accordingly so that the treats are a part of the daily allowance rather than extra to it.
The feeding recommendations on dog food packs always seemed to be rather generous to me - and the more weight the dog puts on the more the recommended allowance goes up. With an adult dog you should be able to feel their ribs but not see them, although the coat on a Golden Retriever tends to obsure the ribs anyway. If you look down on your dog from above he should have a waist that goes in just behind his rib cage and, when viewed from the side, he should tuck up behind his ribs towards his hips.
I would imagine it would be the same with a puppy but hopefully somebody else will be able to confirm/deny that for you.