I would say that is very small on average. I've seen larger litters than 9 in which the puppies go home at 8 weeks at more than double that weight. Lots of possible factors here -- maybe the parents are really small or their lines run small, maybe they aren't purebred (Are you just taking the breeder's word for it?), maybe mom wasn't fed enough during gestation or after whelping, maybe the puppies weren't fed enough after weaning, etc. The puppy might end up being average size or smaller than average. At this point, if the vet says he's healthy, I would just focus on enjoying your puppy and accept that he's going to turn out whatever size he ends up.