Hi there and welcome. First Id like to say that we love to see pics here
I agree with the others, it is better to stay on the thin side than on the heavy side when you are a big or giant breed. It is way better if they grow in a steady and little slower rate then too fast, they will get however big they were meant to be anyway, that is determined by genetics.
Different food can play a roll though, I know that a lot of people like to feed lamb and rice, but if you think that he could weigh a little more, maybe you could try another food just to see if that works any better. Also the recommended cups per day on the bags, are good to have as a general idea, but all dogs are individuals, and maybe he is just one of them who need a little more. Since you have been to the vet, I am sure they took a fecal test, so he has no parasites, that can also be a reason why he isnt putting on wheight.
But if he is a happy puppy, stomach fine and the vet says he is okay, I wouldnt worry, they are just like us people, some of us can eat and eat and not put on weight, others like me have to watch every little thing we eat or we'd be rolling along. If you want to do anything, I would probably try another food just to see. Good Luck and please please post a pic or two for us to awwww over