Will my rotti pup become full sized?

I got my rotti pup from a rescue, they neutered him at around 8 weeks...because they probably don't really care. I took him to the vet today and he says that puppies he sees around this size at this age usually only grow to 30 lbs, he is currently 6.5 lbs, which I know is small for this age, but he doesn't look unhealthy, his coat is shiny and he doesn't look like he's starving or anything. He also estimated my pup to be 7.5 weeks old. Is there anyway that he is just the runt of the litter and will grow to be full size? Or do you think he'll just be a tiny rotti?