Is my GSD growing too fast?

Your pup is large because the parents are MUCH larger than the German Shepherd standard calls for. I don't know much about the "Shiloh" Shepherd, but from what I have seen, they breed over-sized, long haired dogs. ANY breeder that strives for something other than what the breed standard calls for (IMO), is not a good breeder. That being said, your pup is just over-sized because that is what he was bred from.